Polish wording in contribute.md - #911

Changed pages include:

• index.html - view
• 404/index.html - view
• about/index.html - view
• arraycomputing/index.html - view
• case-studies/index.html - view
• citing-numpy/index.html - view
• code-of-conduct/index.html - view
• community/index.html - view
• contribute/index.html - view
• diversity_sep2020/index.html - view

There are more changes not included in the list