Fix doc comment mismatch in vpx_frame_buffer.h
Fix doc comment mismatch in vpx_frame_buffer.h

When compiling an app using libvpx in Xcode 9.2, a warning is
thrown in vpx_frame_buffer.h:

  "Parameter 'new_size' not found in the function declaration"

Switching it to 'min_size' to match the comment text and the
callback type definition prototype resolves it.

Change-Id: I7a3e4a857c2007c2d0d390e22054d7bc85068aa1

ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx