kbuild: doc: update the description about kbuild Makefiles

This line was written in 2003. Now we have much more Makefiles.

The number of Makefiles is not important. The point is we have a
Makefile in (almost) every directory.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Documentation/kbuild/makefiles.rst

@@ -71,7 +71,7 @@ The Makefiles have five parts::
 	.config			the kernel configuration file.
 	arch/$(ARCH)/Makefile	the arch Makefile.
 	scripts/Makefile.*	common rules etc. for all kbuild Makefiles.
-	kbuild Makefiles	there are about 500 of these.
+	kbuild Makefiles	exist in every subdirectory
 
 The top Makefile reads the .config file, which comes from the kernel
 configuration process.