Fix minor typo in 09_bounded.md (#125)

Trim errant `s` from `enqueue`'s end in [Bounded vs unbounded channels](https://rust-exercises.com/100-exercises/07_threads/09_bounded.html):

`if the producers enqueues messages` -> `if the producers enqueue messages`

book/src/07_threads/09_bounded.md

@@ -3,7 +3,7 @@
 So far we've been using unbounded channels.\
 You can send as many messages as you want, and the channel will grow to accommodate them.\
 In a multi-producer single-consumer scenario, this can be problematic: if the producers
-enqueues messages at a faster rate than the consumer can process them, the channel will
+enqueue messages at a faster rate than the consumer can process them, the channel will
 keep growing, potentially consuming all available memory.
 
 Our recommendation is to **never** use an unbounded channel in a production system.\