Correct rerun docs

l3build.dtx

@@ -457,7 +457,7 @@
 % line option |--engine| (or |-e|) has been given to limit
 % testing to a single engine. Normally testing is preceded by unpacking
 % source files and copying the result plus any additional support to the
-% test directory: this may be skipped using the |-s| option.
+% test directory: this may be skipped using the |--rerun| (|-r|) option.
 % \end{buildcmd}
 %
 % \begin{buildcmd}{clean}