Fix link formatting in contributing guide

[3t8]

TL;DR

Fix link formatting contributing guide.

Type

• [ ] Bug Fix
• [ ] Feature
• [ ] Plugin
• [x] Documentation

Are all requirements met?

• [ ] Code completed
• [ ] Smoke tested
• [ ] Unit tests added
• [x] Code documentation added
• [ ] Any pending items have an associated Issue

Complete description

Fixed the first link to flytectl.rtfd.io in the contributing guide. https://docs.flyte.org/projects/flytectl/en/latest/contribute.html#contribute-to-documentation