Fix typos and grammatical errors

[mchu]

Fixes minor typos and grammatical errors in:

• behavioral/README.md
• docs/behavioral.md
• docs/etiquette.md
• docs/index.md
• docs/preparing-mentally.md
• docs/system-design.md
• docs/types-of-interviews.md
• docs/where-should-i-interview.md

The commit should not conflict with the proposed changes in #29 , #31 , and #32.

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Name	Status	Preview	Comments	Updated (UTC)
interview-guide-x545	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Feb 8, 2024 10:22pm