[re.grammar] Fix typeface of 'match_type_flag'

Author: zygoloid

source/regex.tex

@@ -4164,7 +4164,7 @@
 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\iref{re.matchflag} specified when using the regular expression
+to any \tcode{match_flag_type} flags\iref{re.matchflag} specified when using the regular expression
 object in one of the regular expression algorithms\iref{re.alg}. The behavior is also
 localized by interaction with the traits class template parameter as follows:
 \begin{itemize}