README: fix a wrong filename

It should be Documentation/build/kconfig.txt.

Introduced by commit 2af238e4
("kbuild: make *config usage docs").
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

README

@@ -188,7 +188,7 @@ CONFIGURING the kernel:
 			   values to random values.
 
    You can find more information on using the Linux kernel config tools
-   in Documentation/kbuild/make-configs.txt.
+   in Documentation/kbuild/kconfig.txt.
 
 	NOTES on "make config":
 	- having unnecessary drivers will make the kernel bigger, and can