Fix typo chapter 18.2, Quiz Answer chapter 17.4

[AthfanFasee]

I fixed few typos in chapter 18.2 (17-9 -> 18-9, 17-11 -> 18-11)

[AthfanFasee]

Also in Chapter 17.4 Quiz No.2 - The answer (context) should be changed from
"Merging will always respect ordering in each stream, so for example "B" cannot occur before "C".
TO
"Merging will always respect ordering in each stream, so for example "C" cannot occur before "B".

But the contribution rules have mentioned that only src directory should be edited. Also If I try to make the change in the "rust-book\quizzes\async-04-streams.toml" file, the format gets edited as well. Hence this is not included in my PR but I will put it here as a CR.