Update introduction.md

[jcat4]

This fixes a few typos I found in the introduction.

[meatball133]

Changes should also be applied to the following files:

• concept:characters-and-strings:about.md
• exercise:characters-and-strings:introduction.md

Otherwise, the changes look good!

[jcat4]

Alright, I've made the changes to the files you pointed out

[IsaacG]

+cc @exercism/swift