Correct typo

Doc/lib/libsys.tex

@@ -31,7 +31,7 @@ It is always available.
   A triple (repo, branch, version) representing the Subversion
   information of the Python interpreter.
   \var{repo} is the name of the repository, \code{'CPython'}.
-  \var{branch} is the a string of one of the forms \code{'trunk'},
+  \var{branch} is a string of one of the forms \code{'trunk'},
   \code{'branches/name'} or \code{'tags/name'}.
   \var{version} is the output of \code{svnversion}, if the
   interpreter was built from a Subversion checkout; it contains