Maketest.rules.in: Fix comment.

dlls/Maketest.rules.in

 # Global rules for building dll unit tests     -*-Makefile-*-
 #
 # Each individual makefile should define the following variables:
-# DLLTEST      : the dll to test
+# TESTDLL      : the dll to test
 # CTESTS       : list of C test programs
 # EXTRALIBS    : extra libraries to link in (optional)
 # EXTRADEFS    : extra symbol definitions, like -DWINELIB (optional)