Update README-fr.md (ziishaned#89)

* Update README-fr.md

hi, i'm ahmed, an french student, who learned regex.
I wanted correct some mistakes like the lack of plurial in some expressions, and bad linking into 2 words.
I corrected also the expression of string because in France for someone who don't know well computer science , the "string" means a clothes that people wear.
I checked the file and everything is okay.
I will check again tomorrow.
I hope help you again.
Ahmed

* Update README-fr.md

added  (string) after "chaines de caracteres"

Author: ahmedaboukora