Typo, avoid ligature in long option name

Somehow lmodern package enables the "--" (double-hyphen) to en-dash
ligature for \ttfamily, in T1 encoding. The result looks wrong for
long CLI options.

l3build.dtx

@@ -388,7 +388,7 @@
 % \item |--message| (|-m|) Text for upload announcement
 % \item |--quiet| (|-q|) Suppresses output from unpacking
 % \item |--rerun| Runs tests without unpacking/set up
-% \item |--show-log-on-error| To be used in addition to \texttt{--halt-on-error} and results
+% \item |--show-log-on-error| To be used in addition to |--halt-on-error| and results
 %   in the full \texttt{.log} file of a failed test to be shown on the console
 % \item |--show-saves| (|-S|) When tests fail, prints the \texttt{l3build save} commands needed
 %   to regenerate the tests assuming that the failures were false negatives.