kbuild/Documentation: Incorrect makefile syntax in example

There is an error in the make syntax for one of the kbuild examples
Signed-off-by: NDavid VomLehn <dvomlehn@cisco.com>
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>

Documentation/kbuild/modules.txt

@@ -275,7 +275,7 @@ following files:
 
 		KERNELDIR := /lib/modules/`uname -r`/build
 		all::
-			$(MAKE) -C $KERNELDIR M=`pwd` $@
+			$(MAKE) -C $(KERNELDIR) M=`pwd` $@
 
 		# Module specific targets
 		genbin: