Fix broken prost-build link.

This appears to reference a section in the markdown that no longer exists.
Reference the docs.rs page instead.

README.md

@@ -36,11 +36,13 @@ prost-types = "0.12"
 ```
 
 The recommended way to add `.proto` compilation to a Cargo project is to use the
-`prost-build` library. See the [`prost-build` documentation](prost-build) for
+`prost-build` library. See the [`prost-build` documentation][prost-build] for
 more details and examples.
 
-See the [snazzy repository](https://github.com/danburkert/snazzy) for a simple
-start-to-finish example.
+See the [snazzy repository][snazzy] for a simple start-to-finish example.
+
+[prost-build]: https://docs.rs/prost-build/latest/prost_build/
+[snazzy]: https://github.com/danburkert/snazzy
 
 ### MSRV