model/guild_channel: revert 'say' taking self

Revert the 'say' method in the impl of 'guildchannel' from taking self.
This was accidentally done in a Clippy lint PR[1].

This is a breaking change, but is a bugfix, so a patch increase is fine.

[1]: #579

Signed-off-by: Zeyla Hellyer <zeyla@hellyer.dev>

src/model/channel/guild_channel.rs

@@ -628,7 +628,7 @@ impl GuildChannel {
     /// [`ModelError::MessageTooLong`]: ../error/enum.Error.html#variant.MessageTooLong
     #[cfg(feature = "http")]
     #[inline]
-    pub fn say(self, http: impl AsRef<Http>, content: impl std::fmt::Display) -> Result<Message> { self.id.say(&http, content) }
+    pub fn say(&self, http: impl AsRef<Http>, content: impl std::fmt::Display) -> Result<Message> { self.id.say(&http, content) }
 
     /// Sends (a) file(s) along with optional message contents.
     ///