doc typo fix.
doc typo fix.

* subversion/include/svn_error.h
 (svn_error_purge_tracing): Remove extraneous 'is' from the docstring.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion