remove extra word occurance; reported to webmaster

Doc/lib/liburllib.tex

@@ -146,7 +146,7 @@ The public functions \function{urlopen()} and
 \class{FancyURLopener} class and use it to perform their requested
 actions.  To override this functionality, programmers can create a
 subclass of \class{URLopener} or \class{FancyURLopener}, then assign
-that an instance of that class to the
+an instance of that class to the
 \code{urllib._urlopener} variable before calling the desired function.
 For example, applications may want to specify a different
 \mailheader{User-Agent} header than \class{URLopener} defines.  This