Update doctests for Context changes

Due to the Context having many methods removed, the doctests were
failing.

Update the doctests to use alternative methods to accomplish the same.

Example:

Before:
```rust
context.say("hi");
```

After:
```rust
message.channel_id.say("hi")
```

1 files changed, 1 insertions, 1 deletions

diff --git a/src/client/mod.rs b/src/client/mod.rs
index d0593fe..0da4b0e 100644
--- a/src/client/mod.rs
+++ b/src/client/mod.rs
@@ -138,7 +138,7 @@ lazy_static! {
 ///
 /// client.on_message(|context, message| {
 ///     if message.content == "!ping" {
-///         context.say("Pong!");
+///         message.channel_id.say("Pong!");
 ///     }
 /// });
 ///