Correct an error message.  No functional change.

* subversion/libsvn_subr/types.c
  (svn_revnum_parse): we can't have leading '0' in that case


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1489826 13f79535-47bb-0310-9956-ffa450edef68