Fix typo in Chapter 10 traits example output

[amsorrytola]

Issue

In Chapter 10.2 under the section Implementing a Trait on a Type of The Rust Book, the text says:

This code prints 1 new post: horse_ebooks: of course, as you probably already know, people.

But the actual code uses:

println!("1 new social post: {}", post.summarize());

This creates a mismatch between the expected and actual output. The word “social” is missing in the textual explanation.

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This PR fixes

• Updates the output description to match the format string in the code example.
• Corrected sentence:

This code prints 1 new social post: horse_ebooks: of course, as you probably already know, people.

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📄 Notes

• Only one line is modified in src/ch10-02-traits.md.
• No formatting or structural changes elsewhere.