chore: revise developer guide

Updates developer guide to be more clear how to get up and going in development with gitiles. Was confusing to start developing because the section said "tests", not "developing" or "Locally using" or something similar.

Change-Id: Icb2f8a349cdad52fd30cd7795665183e26e0a3a2
diff --git a/Documentation/developer-guide.md b/Documentation/developer-guide.md
index 516dac0..08d3f46 100644
--- a/Documentation/developer-guide.md
+++ b/Documentation/developer-guide.md
@@ -10,12 +10,12 @@
 bazel.build: https://bazel.build/versions/master/docs/install.html
 
 ```
-  bazel build //...
+  bazel build //:gitiles
   bazel test //...
 ```
 
 
-## Testing
+## Running Locally and Testing
 
 ```
 cd /path/to/repositories  # Don't run from the gitiles repo.