docs: Kconfig: fix defconfig description

Commit 2a86f66 ("kbuild: use KBUILD_DEFCONFIG as the fallback
for DEFCONFIG_LIST") removed arch/$ARCH/defconfig; however,
the document has not been updated to reflect this change yet.

Signed-off-by: Satoru Takeuchi <satoru.takeuchi@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Author: satoru-takeuchi

Documentation/admin-guide/README.rst

@@ -176,7 +176,7 @@ Configuring the kernel
                         values without prompting.
 
      "make defconfig"   Create a ./.config file by using the default
-                        symbol values from either arch/$ARCH/defconfig
+                        symbol values from either arch/$ARCH/configs/defconfig
                         or arch/$ARCH/configs/${PLATFORM}_defconfig,
                         depending on the architecture.