Slightly improved indexing for the string-% operator, thanks to comments

from Skip Montanaro.  There is one weirdness in the final index for HTML, but
that is low priority.

Doc/lib/libstdtypes.tex

@@ -670,10 +670,11 @@ Return a copy of the string converted to uppercase.
 
 \subsubsection{String Formatting Operations \label{typesseq-strings}}
 
-\index{formatting, string}
+\index{formatting, string (\%{})}
 \index{string!formatting}
 \index{printf-style formatting}
 \index{sprintf-style formatting}
+\index{\protect\%{} formatting}
 
 String and Unicode objects have one unique built-in operation: the
 \code{\%} operator (modulo).  Given \code{\var{format} \%