fix typo (facebook#24343)

Summary:
"Built from source" is past tense - if anything, it should be "Build from source". However, all other heading end with "ing" so it makes sense here too.
Pull Request resolved: facebook#24343

Differential Revision: D14822098

Pulled By: cpojer

fbshipit-source-id: 3d07f2e6f8ed4a21e0311ef4f675f2d41553b619

Author: vonovak

RNTester/README.md

@@ -81,7 +81,7 @@ When developing E2E tests, you may want to run in development mode, so that chan
 
 You will also need to have Metro Bundler running in another terminal. Note that if you've previously run the E2E tests in release mode, you may need to delete the `RNTester/build` folder before rerunning `detox build`.
 
-## Built from source
+## Building from source
 
 Building the app on both iOS and Android means building the React Native framework from source. This way you're running the latest native and JS code the way you see it in your clone of the github repo.