[re.regex.construct] Add missing default argument to itemdecl (#1893)

Author: jwakely

source/regex.tex

@@ -1478,7 +1478,7 @@
 
 \indexlibrary{\idxcode{basic_regex}!constructor}%
 \begin{itemdecl}
-basic_regex(const charT* p, size_t len, flag_type f);
+basic_regex(const charT* p, size_t len, flag_type f = regex_constants::ECMAScript);
 \end{itemdecl}
 
 \begin{itemdescr}