Fix missing word

As reported in https://github.com/redox-os/redox/issues/629

Typo =)

Update shell.md

Many tweaks to the Redox Book

The changes include:
- Editing for flow and understandability.
- Unifying how links are included in the source files.
- Adding an `About this Book` page to the introduction.
- Adding a few examples or new pieces of information I felt were
  lacking when I read the book.
- Unifying 'user space' and 'kernel space' spelling.
- Added myself to the authors list in `overview/welcome.md`.

update preparing_the_build.md & compiling_redox.md

Fix minor spelling and grammatical issues

Minor edits to first 2 chapters

Update what_is_redox.md

Fix typo

Go into more detail and reason about micro kernels

Changes to try and improve public relations

Conflicts:
	src/overview/developers.md

Add some links to developers.md file

Expand "Everything is an URL" section