Update l3build.dtx

Minor improvement

l3build.dtx

@@ -569,7 +569,7 @@
 % ISO format (YYYY-MM-DD). If no \meta{tag name} is given, the tag will default to |nil|.
 % Both are passed as arguments to the |update_tag()| function.
 %
-% The standard setup does nothing unless tag update is set up by defining |update_tag()|. See Section~\ref{sec:tagging} for full details on this feature.
+% The standard setup does nothing unless tag update is set up by defining a custom |update_tag()| function. See Section~\ref{sec:tagging} for full details on this feature.
 % \end{buildcmd}
 %
 % \begin{buildcmd}{unpack}