Docstring fix.  No functional change.

* subversion/include/private/svn_subr_private.h
  (svn__fnv1a_32,
   svn__fnv1a_32x4): While the digest uses big-endian ordering,
                     the runtime integer values are in machine order.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1606139 13f79535-47bb-0310-9956-ffa450edef68