arch/xtensa: fix grammar in Kconfig help text

Spell "Don't" correctly in the XTENSA_VARIANT_CUSTOM_NAME help text.
Signed-off-by: Hu Haowen <xianfengting221@163.com>
Message-Id: <20200330045436.12645-1-xianfengting221@163.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/Kconfig

@@ -122,7 +122,7 @@ config XTENSA_VARIANT_CUSTOM_NAME
 	help
 	  Provide the name of a custom Xtensa processor variant.
 	  This CORENAME selects arch/xtensa/variant/CORENAME.
-	  Dont forget you have to select MMU if you have one.
+	  Don't forget you have to select MMU if you have one.
 
 config XTENSA_VARIANT_NAME
 	string