Follow-up to r1639358: Update docstring.

* subversion/libsvn_fs_fs/revprops.c
  (switch_to_new_revprop): Do not mention removed BUMP_GENERATION argument 
   in docstring.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1639419 13f79535-47bb-0310-9956-ffa450edef68