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>