docs: correct exportToBackend to onExportToBackend in README (#3952)

* docs: correct exportToBackend to onExportToBackend in README

* Update changelog with link to PR

* Add Excalidraw API title to changelog section

Co-authored-by: Aakansha Doshi <aakansha1216@gmail.com>

Co-authored-by: Aakansha Doshi <aakansha1216@gmail.com>

src/packages/excalidraw/CHANGELOG.md

 
 - Prevent gradual misalignment of the canvas due to floating point rounding errors [#3833](https://github.com/excalidraw/excalidraw/pull/3833).
 
+## Excalidraw API
+
+### Docs
+
+- Correct exportToBackend in README to onExportToBackend [#3952](https://github.com/excalidraw/excalidraw/pull/3952)
+
 ## 0.9.0 (2021-07-10)
 
 ## Excalidraw API

src/packages/excalidraw/README.md

 | Attribute | Type | Default | Description |
 | --- | --- | --- | --- |
 | `saveFileToDisk` | boolean | true | Implies if save file to disk button should be shown |
-| `exportToBackend` | <pre> (exportedElements: readonly NonDeletedExcalidrawElement[],appState: AppState,canvas: HTMLCanvasElement &#124; null) => void </pre> |  | This callback is triggered when the shareable-link button is clicked in the export dialog. The link button will only be shown if this callback is passed. |
+| `onExportToBackend` | <pre> (exportedElements: readonly NonDeletedExcalidrawElement[],appState: AppState,canvas: HTMLCanvasElement &#124; null) => void </pre> |  | This callback is triggered when the shareable-link button is clicked in the export dialog. The link button will only be shown if this callback is passed. |
 | `renderCustomUI` | <pre> (exportedElements: readonly NonDeletedExcalidrawElement[],appState: AppState,canvas: HTMLCanvasElement &#124; null) => void </pre> |  | This callback should be supplied if you want to render custom UI in the export dialog. |
 
 #### `onPaste`

src/packages/excalidraw/README_NEXT.md

 | Attribute | Type | Default | Description |
 | --- | --- | --- | --- |
 | `saveFileToDisk` | boolean | true | Implies if save file to disk button should be shown |
-| `exportToBackend` | <pre> (exportedElements: readonly NonDeletedExcalidrawElement[],appState: AppState,canvas: HTMLCanvasElement &#124; null) => void </pre> |  | This callback is triggered when the shareable-link button is clicked in the export dialog. The link button will only be shown if this callback is passed. |
+| `onExportToBackend` | <pre> (exportedElements: readonly NonDeletedExcalidrawElement[],appState: AppState,canvas: HTMLCanvasElement &#124; null) => void </pre> |  | This callback is triggered when the shareable-link button is clicked in the export dialog. The link button will only be shown if this callback is passed. |
 | `renderCustomUI` | <pre> (exportedElements: readonly NonDeletedExcalidrawElement[],appState: AppState,canvas: HTMLCanvasElement &#124; null) => void </pre> |  | This callback should be supplied if you want to render custom UI in the export dialog. |
 
 #### `onPaste`