kbuild: update ARCH alias info for sparc

The supported alias for building sparc 32-bit is "sparc32",
not "sparc", so update the alias documentation for that.
Just using "sparc" produces a 64-bit config file.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Documentation/kbuild/kbuild.txt

@@ -89,7 +89,7 @@ directory name found in the arch/ directory.
 But some architectures such as x86 and sparc have aliases.
 x86: i386 for 32 bit, x86_64 for 64 bit
 sh: sh for 32 bit, sh64 for 64 bit
-sparc: sparc for 32 bit, sparc64 for 64 bit
+sparc: sparc32 for 32 bit, sparc64 for 64 bit
 
 CROSS_COMPILE
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