Fix grammatical/spelling errors in UG

[weiseng18]

This PR fixes the grammatical and spelling errors raised in PE-D.

Only these errors are fixed to ease the load on reviewing, since there are many separate issues that need to be verified.

Listed in order of commit:

• Closes #279
• Closes #253
• Closes #255
• Closes #301
• Closes #294
• Closes #276
• Closes #263

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark: