Fix spelling in docstrings.

* subversion/include/private/svn_mutex.h
  (): s/Strutures/Structures/
  (svn_mutex__lock,
   svn_mutex__unlock,
   SVN_MUTEX__WITH_LOCK): s/aquisition/acquisition/


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1481418 13f79535-47bb-0310-9956-ffa450edef68