[re.grammar] Fix typeface of 'match_type_flag'

Author: zygoloid

source/regex.tex

@@ -4166,7 +4166,7 @@
 The behavior of the internal finite state machine representation when used to match a
 sequence of characters is as described in ECMA-262.
 The behavior is modified according
-to any match_flag_type flags~(\ref{re.matchflag}) specified when using the regular expression
+to any \tcode{match_flag_type} flags~(\ref{re.matchflag}) specified when using the regular expression
 object in one of the regular expression algorithms~(\ref{re.alg}). The behavior is also
 localized by interaction with the traits class template parameter as follows:
 \begin{itemize}