kbuild/Documentation: Incorrect makefile syntax in example

There is an error in the make syntax for one of the kbuild examples

Signed-off-by: David VomLehn <dvomlehn@cisco.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/Documentation/kbuild/modules.txt b/Documentation/kbuild/modules.txt
index b1096da..0767cf6 100644
--- a/Documentation/kbuild/modules.txt
+++ b/Documentation/kbuild/modules.txt
@@ -275,7 +275,7 @@
 
 		KERNELDIR := /lib/modules/`uname -r`/build
 		all::
-			$(MAKE) -C $KERNELDIR M=`pwd` $@
+			$(MAKE) -C $(KERNELDIR) M=`pwd` $@
 
 		# Module specific targets
 		genbin: