Update README.md (google#9576)

While working on docs, I found that this link was broken. I have changed
it to the correct URL.

docs/README.md

@@ -15,5 +15,5 @@ $ bundle exec jekyll serve
 ```
 
 ## Theme documentation
-We are using the [just the docs](https://pmarsceill.github.io/just-the-docs/)
+We are using the [just the docs](https://just-the-docs.github.io/just-the-docs/)
 theme.