fix truly evil markup typo

Doc/lib/libstring.tex

@@ -221,7 +221,7 @@ The functions defined in this module are:
   words is the same as returned by \function{split()}, except when the
   optional third argument \var{maxsplit} is explicitly specified and
   nonzero.  When \var{maxsplit} is nonzero, at most \var{maxsplit}
-  number of splits -- the \em{rightmost} ones -- occur, and the remainder
+  number of splits -- the \emph{rightmost} ones -- occur, and the remainder
   of the string is returned as the first element of the list (thus, the
   list will have at most \code{\var{maxsplit}+1} elements).
   \versionadded{2.4}