kconfig: gconfig: correct program name in help text

Change "gkc" to "gconfig" in 3 places since it is called "gconfig" and
not "gkc". Add a period at the end of one sentence.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>