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 <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Max Filippov <[email protected]>

Author: srcres2581

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