langref: fix missing word typo

doc/langref.html.in

@@ -370,7 +370,7 @@ pub fn main() !void {
 }
       {#code_end#}
       <p>
-      Most of the time, it more appropriate to write to stderr rather than stdout, and
+      Most of the time, it is more appropriate to write to stderr rather than stdout, and
       whether or not the message is successfully written to the stream is irrelevant.
       For this common case, there is a simpler API:
       </p>