Change pull.md title to "Pull API".
Change pull.md title to "Pull API".

This makes the title more consistent in the context of the other docs, and
avoids confusion around callbacks used for other current (or future) APIs.

librsync/librsync,sourcefrog/librsync,sourcefrog/librsync,librsync/librsync,librsync/librsync,librsync/librsync,dbaarda/librsync,dbaarda/librsync,sourcefrog/librsync,dbaarda/librsync,sourcefrog/librsync,dbaarda/librsync