Followup to r21738, correcting a typo in a comment.
Followup to r21738, correcting a typo in a comment.

* subversion/libsvn_fs_fs/fs_fs.c
  (get_writable_proto_rev): Correct the name of an error that can be returned.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@861835 13f79535-47bb-0310-9956-ffa450edef68

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