Fix a docstring.

* subversion/libsvn_client/client.h
  (svn_client__checkout_internal): innercheckout isn't an argument remove the
    obsolete doccstring.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1561257 13f79535-47bb-0310-9956-ffa450edef68