Fix access_token description in revokeChannelToken (#130)

There was an openapi-generator bug
(OpenAPITools/openapi-generator#24004, fixed in v7.24.0 via
OpenAPITools/openapi-generator#24192).

It generated `revokeChannelToken`'s `access_token` doc as `A short-lived
or long-lived channel access token.` instead of this spec's `Channel
access token`, and the SDKs merged it.

With v7.24.0, regeneration now tries to revert it:

- line/line-bot-sdk-ruby#877
- line/line-bot-sdk-nodejs#1751
- line/line-bot-sdk-php#879

Since the current SDK wording is correct for this endpoint (`POST
/v2/oauth/revoke` only accepts short-lived or long-lived tokens), this
PR updates the spec to match it instead.

-
https://developers.line.biz/en/reference/messaging-api/#revoke-longlived-or-shortlived-channel-access-token
- The endpoint section is titled "Revoke short-lived or long-lived
channel access token".

The regeneration diff disappears and the PRs above stay closed.

Author: mokuzon

channel-access-token.yml

@@ -284,7 +284,7 @@ paths:
               type: object
               properties:
                 access_token:
-                  description: "Channel access token"
+                  description: "A short-lived or long-lived channel access token."
                   type: string
               required:
                 - access_token