Fix caveats in protobuf

The caveats for the current formula list
/usr/local/Cellar/protobuf/2.5.0/share/doc/protobuf/protobuf as the
directory containing examples and editor support. The proper directory
is /usr/local/Cellar/protobuf/2.5.0/share/doc/protobuf

Closes #18521.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>