Fix #1676656: \em is different from \emph...

Doc/lib/liburlparse.tex

@@ -168,7 +168,7 @@ for \function{urlparse()}.
       
 If you do not want that behavior, preprocess
 the \var{url} with \function{urlsplit()} and \function{urlunsplit()},
-removing possible \em{scheme} and \em{netloc} parts.
+removing possible \emph{scheme} and \emph{netloc} parts.
 \end{funcdesc}
 
 \begin{funcdesc}{urldefrag}{url}