how_to_handle_statics.rst: Update make command

`make css` has been renamed to `make assets`.

docs/source/howto/how_to_handle_statics.rst

 local development, :command:`npm run watch` will watch for local changes to SASS files and
 automatically rebuild the compiled CSS.
 
-Use the command :command:`make css` to compile assets manually.
+Use the command :command:`make assets` to compile assets manually.
 
 Javascript
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