Merge pull request #15576 from grafana/fix/yarn-docs

Update docs to match current npm scripts

docs/sources/project/building_from_source.md

@@ -83,7 +83,7 @@ go get github.com/Unknwon/bra
 bra run
 ```
 
-You'll also need to run `yarn watch` to watch for changes to the front-end (typescript, html, sass)
+You'll also need to run `yarn start` to watch for changes to the front-end (typescript, html, sass)
 
 ### Running tests