kbuild: get rid of trailing slash from subdir- example

obj-* needs a trailing slash for a directory, but subdir-* does not.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Documentation/kbuild/makefiles.rst

@@ -765,7 +765,7 @@ is not sufficient this sometimes needs to be explicit.
 	Example::
 
 		#arch/x86/boot/Makefile
-		subdir- := compressed/
+		subdir- := compressed
 
 The above assignment instructs kbuild to descend down in the
 directory compressed/ when "make clean" is executed.