Minor grammatical fix

Doc/lib/libstring.tex

@@ -142,7 +142,7 @@ The functions defined in this module are:
   Return the number of (non-overlapping) occurrences of substring
   \var{sub} in string \code{\var{s}[\var{start}:\var{end}]}.
   Defaults for \var{start} and \var{end} and interpretation of
-  negative values is the same as for slices.
+  negative values are the same as for slices.
 \end{funcdesc}
 
 \begin{funcdesc}{lower}{s}