common: Fix comment for saveenv in Kconfig

The help for this was simply copied from another command.

Update it to reflect the command.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/common/Kconfig b/common/Kconfig
index f6478fa..0ccdf62 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -188,7 +188,8 @@
 config CMD_SAVEENV
 	bool "saveenv"
 	help
-	  Run the command in the given environment variable.
+	  Save all environment variables into the compiled-in persistent
+	  storage.
 
 endmenu