Fix __atomic_store_n typo in recent manual change

gcc/:
2013-06-22  Andi Kleen  <ak@linux.intel.com>

       * doc/extend.texi: Use __atomic_store_n instead of
       __atomic_store in HLE example.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@200333 138bc75d-0d04-0410-961f-82ee72b054a4