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: Anton Blanchard <anton@samba.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/Documentation/kbuild/modules.txt b/Documentation/kbuild/modules.txt
index 769ee05..1d247d5 100644
--- a/Documentation/kbuild/modules.txt
+++ b/Documentation/kbuild/modules.txt
@@ -249,7 +249,7 @@
 		--> filename: Makefile
 		KERNELDIR := /lib/modules/`uname -r`/build
 		all::
-			$(MAKE) -C $KERNELDIR M=`pwd` $@
+			$(MAKE) -C $(KERNELDIR) M=`pwd` $@
 
 		# Module specific targets
 		genbin: