Chapter 10-2: Implementing a Trait on a Type: `declared` should be used instead of `defined` in first line.

[asher-gh]

• I have checked the latest main branch to see if this has already been fixed
• I have searched existing issues and pull requests for duplicates

URL to the section(s) of the book with this problem:
https://doc.rust-lang.org/book/ch10-02-traits.html#implementing-a-trait-on-a-type

Description of the problem:
The first line of the para says:

Now that we’ve defined the desired behaviour using the Summary trait, we can implement it on the types in our media aggregator.

Suggested fix:
I feel declared would be a better term as we haven't defined the behaviour but only the trait.

[carols10cents]

Thanks! I decided to go with "define the signatures" instead, because I think we have defined something, but you're right that it's not the behavior.