docs(ReactionManager): clarify cache Collection keys type

[ghost]

Please describe the changes this PR makes and why it should be merged:
ReactionManager#cache returns a collection of MessageReaction mapped by either a Snowflake (for custom emojis) or a string(for emojis); but current type of ReactionManager#cache is Collection<Snowflake, MessageReaction>.
Even if Snowflake is just a named typedef for string, it still seems a bit misleading and semantically incorrect.

Status

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.

[vaporoxx]

Snowflake is typed as just string, so Collection<string, MessageReaction> would be sufficient in my opinion. Also, you would need to change the typings here (i.e. BaseManager<string, ...) before the "Updated typings" box can be checked.

[ghost]

Also, you would need to change the typings here (i.e. BaseManager<string, ...) before the "Updated typings" box can be checked.

uh, I didn't knew I had to do it. Done, thank you.