[strings.general] Use plural "string classes" (#1823)

The summary even says "string classes", and now we also have the "string_view classes".

Author: JohelEGP

source/strings.tex

@@ -13,7 +13,7 @@
 
 \pnum
 The following subclauses describe a
-character traits class, a string class, and
+character traits class, string classes, and
 null-terminated sequence utilities,
 as summarized in Table~\ref{tab:strings.lib.summary}.