Fix vector/array/slice terminology in manual.

fixes #16015
Could you include arrays and slices in backticks (just for readability).

I'm not sure how much precedent we have in the manual for this, but you may want to explicitly point out that `Vec<T>` is *not* part of the language, only arrays and slices are. As a user of rust I should be aware of all three, but as a theoretical compiler implementer there's only two