docs: Kconfig: fix defconfig description

Commit 2a86f6612164 ("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 <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>

Documentation/admin-guide/README.rst

···
                         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.