Fix Doxygen mark-up.

* subversion/include/svn_config.h
  (svn_config_get_int64): s/@cfg/@a cfg/.

* subversion/include/svn_io.h
  (svn_io_start_cmd3): s/@env/@a env/.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1463747 13f79535-47bb-0310-9956-ffa450edef68