Note to self: next time, try to make a trivial documentation change in one go.

* subversion/include/svn_ra.h (svn_ra_open_tunnel_func_t):
   Fix typo and tweak the docstring a bit more.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1560807 13f79535-47bb-0310-9956-ffa450edef68