Fix a link to CustomTabsCallback in support-library docs.

Bug:
https://code.google.com/p/android/issues/detail?id=183077

Change-Id: I86d1d8e07e42e9e295364b002e7e1534d4d9d5cc
(cherry picked from commit f82032fcf93a2cfea24574632cca676af6bc83fa)
1 file changed