Fix typo in introduction.md in freelance-rates exercise

Fixed a couple of typos in the introduction to Freelance-Rates exercise.
Pull Request

It fixes a couple more that I found after the first commit. The pr won’t update I guess since it’s closed.

There is another thread about fixing these, but didn’t actually fix this one.
Mentioned here and here

I found these after already making my fork and trying to push.

Thanks for helping out. I reopened your pr. please submit as many typos as you can in one PR, so it is not too crowded

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Updated with a couple more fixes. Ready for review now.