Typo and grammar mistakes corrected in mentor.rst

Please review and accept this merge request. I've corrected various typos and grammar mistakes in the mentor.rst file. Following changes have been made:

• mentors and contributors to best service the community ---> mentors and contributors to best serve the community
• Don’t be shy and don’t be offended when we edit. We are thrilled to have you on-board! ---> Don’t be shy, and don’t be offended when we edit. We are thrilled to have you on board!
• Prospective mentors, sudents will use our ---> Prospective mentors, students will use our
• Submit projects ideas ---> Submit project ideas
• also provide guidance to GSoC ---> also provides guidance to GSoC
• You will also need be invited ---> You will also need to be invited

Thank you!