party-robot: tweak introduction

[ErikSchierboom]

The introduction.md document of the party-robot exercise could use some tweaks.

First, the introduction mentions the use of fmt.Printf, but the exemplar solution shows that the student does not need to know about this function to solve the exercise. As per the docs, this means the fmt.Printf mentions should be removed from the introduction. The fmt.Sprintf function does have to be mentioned in the introduction, as that is required.

Secondly, the link to the fmt package should be removed, as we generally don't want to link to external resources in the introduction.