Fix formatting and grammar in CONTRIBUTING.md

[keji-sketch]

Corrected the following phrases: 1. "a new entry in table of contents" 2. "Links must be pointing straight to the tutorials" 3. "Pull request for each of the tutorial"

Description

Motivation and Context

How Has This Been Tested?

Types of changes

• Content Update (change which fixes an issue or updates an already existing submission)
• New Article (change which adds functionality)
• Documentation change

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have made checks to ensure URLs and other resources are valid

[Zentarious]

Looks good to me.