Fixed typos and language in documentation

[volkm]

Thanks for the great introduction into PlusCal and TLA+. The tutorial was very useful for me.
While reading it, I occasionally encountered some typos and other minor language issues which try to fix with this PR.

• Fixed a couple of typos, added missing words or removed some words.
• Consistently add whitespace around em-dash. If a different style is preferred, I can change it again.