Fix Invalid Session opcode description

[Zoddo]

As noticed during a conversation on the Discord API server, there is currently an inconsistency between the "Opcodes and Status Codes" and "Gateway" pages.

The former says that we should reconnect on OP9 before reidentifying/resuming, while the later doesn't mention this. The end of the Gateway#resuming section even says that we should only wait a random amount of time before identifying, and doesn't mention reconnecting.

This inconsistency was introduced by #1373 (a user-made rewording/formatting cleanup of this page).

On our bot, we are always reidentifying/resuming over the existing gateway connection without any issue. I've also looked at Eris, which also reidentify over the existing connection.

Therefore I believe that reconnecting is actually not necessary. The wording I'm removing has likely been added by error in the aforementioned PR.

[Zoddo]

Closing this PR because this info is now accurate with the upcoming change with the resume_gateway_url: https://github.com/discord/discord-api-docs/pull/5282#issuecomment-1215405806