docs(mdbook): minor typo fixes

[mike-vondy]

Was reading through the mdbook to get a grasp of the systems and noticed a small typo.

Typo thought to though in Chapter 2: Creating Inputs.

In our first case, we've said we have an input but we haven't provided any clues about the type of data, and must do so to compile the program even thought we don't care for the example.

Edit: Found another typo as I was going along so I figured I'd include it.
Added an ly to fix relative in mdbook/src/chapter_4/chapter_4_4.md

The capture_into method is relatively simple, and we can just look at it: