Fix typo in the documentation

Author: Antoine Fontaine

include/boost/parser/parser.hpp

@@ -7811,7 +7811,7 @@ namespace boost { namespace parser {
         control;
 
     /** The punctuation character parser.  Matches the full set of Unicode
-        punctuation clases (specifically, "Pc", "Pd", "Pe", "Pf", "Pi", "Ps",
+        punctuation classes (specifically, "Pc", "Pd", "Pe", "Pf", "Pi", "Ps",
         and "Po"). */
     inline BOOST_PARSER_ALGO_CONSTEXPR
         parser_interface<char_set_parser<detail::punct_chars>>