chore(docs): Update GRID app URL

Now we're out of alpha we've moved to a stable app.grid.is sub-domain.

Author: mattgrid

README.md

@@ -4,7 +4,7 @@
 
 This library provides convenient access to the GRID REST API from server-side TypeScript or JavaScript.
 
-The REST API documentation can be found on [docs.grid.is](https://docs.grid.is/). The full API of this library can be found in [api.md](api.md). Your spreadsheets dashboard can be found at [alpha.grid.is](https://alpha.grid.is/).
+The REST API documentation can be found on [docs.grid.is](https://docs.grid.is/). The full API of this library can be found in [api.md](api.md). Your spreadsheets dashboard can be found at [app.grid.is](https://app.grid.is/).
 
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