Update instructions.md

[sparky9043]

Change wording to clarify that there are 22 billable days per month and the days given for each project are all billable days

[github-actions]

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[iHiD]

(This has come out of a conversation with me)

[Cool-Katt]

I was going to mention that this is the second instruction clarification for this exercise in a span of two weeks, and suggest we put the new instructions in problem-spec but I don't think this exercise exists there (or if it does, it's with a different name?)
@iHiD