test-largefiles.t: fix wrong '#if hgweb' - it should be '#if serve'

A bug introduced in 0c1d10351869 did that a part of the test never was run.

tests/test-largefiles.t

@@ -217,7 +217,7 Test copies and moves from a directory o
   ./foo
   $ cd ../../a
 
-#if hgweb
+#if serve
 Test display of largefiles in hgweb
 
   $ hg serve -d -p $HGPORT --pid-file ../hg.pid