talk/objc/README: s/libjingle/webrtc/ in repository path.
talk/objc/README: s/libjingle/webrtc/ in repository path.

TBR=wu@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/1985004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 825e9b0a9b638aff124c8f79e7ac081ecb0df2d1

sippet/talk,sippet/talk,sippet/talk,sippet/talk,sippet/talk