Fix wording in introduction.md

[patrickjaberg]

The change is probably best described by the diff. :-)

[lpil]

This was right before I'm afraid. There are 2 functions that can be used to create Regex objects

[evanasse]

Should it be "The regular expression creation functions return an error...", then?
The word "return" is missing in the sentence.

[lpil]

Thank you both!