Fix README header for subject agreement

This commit changes the "Develop Swift in Xcode" header to be
"Developing Swift in Xcode" to agree with the other headers
in the same file:
 - "Getting Started"
 - "Getting Sources"
 - "Building Swift"
 - "Testing Swift"
 - "Contributing to Swift"

Author: Terrence Katzenbaer

README.md

@@ -116,7 +116,7 @@ A basic command to build Swift and run basic tests with Ninja:
 
     utils/build-script -t
 
-## Develop Swift in Xcode
+## Developing Swift in Xcode
 
 The Xcode IDE can be used to edit the Swift source code, but it is not currently
 fully supported as a build environment for SDKs other than OS X. If you'd like