Correct a doc typo (fixes #368)

l3build.dtx

@@ -1953,7 +1953,7 @@
 %     |tex(|\meta{file},\meta{dir},\meta{cmd}|)|
 %   \end{syntax}
 %   Runs \meta{cmd} (by default \luavar{typesetexe} \luavar{typesetopts}) on the
-%   \meta{name} inside the \meta{dir}.
+%   \meta{file} inside the \meta{dir}.
 % \end{function}
 %
 % \begin{function}{runcmd()}