doc: fix dev server backend default

doc/development.md

 make dev
 ```
 
-By default the backend deployment used is `beta.meet.jit.si`. You can point the Jitsi-Meet app at a different backend by using a proxy server. To do this, set the WEBPACK_DEV_SERVER_PROXY_TARGET variable:
+By default the backend deployment used is `alpha.jitsi.net`. You can point the Jitsi-Meet app at a different backend by using a proxy server. To do this, set the WEBPACK_DEV_SERVER_PROXY_TARGET variable:
 ```
 export WEBPACK_DEV_SERVER_PROXY_TARGET=https://your-example-server.com
 make dev