kbuild: small documentation fix in Documentation/kbuild/modules.txt

The Makefile fragment in Documentation/kbuild/modules.txt looks to be
missing some braces.
Signed-off-by: NAnton Blanchard <anton@samba.org>
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>

Documentation/kbuild/modules.txt

@@ -249,7 +249,7 @@ following files:
 		--> filename: Makefile
 		KERNELDIR := /lib/modules/`uname -r`/build
 		all::
-			$(MAKE) -C $KERNELDIR M=`pwd` $@
+			$(MAKE) -C $(KERNELDIR) M=`pwd` $@
 
 		# Module specific targets
 		genbin: