docs: improve grammar in README (replaced & with and)

[Priyanshu-1477]

Summary

Improves readability in the README by replacing "&" with "and" and removing an unnecessary filler word for clarity.

Before

HTTPie is designed for testing, debugging, and generally interacting with APIs & HTTP servers.

After

HTTPie is designed for testing, debugging, and interacting with APIs and HTTP servers.

Notes

• This is a docs-only change.
• No functional or code behavior was modified.

[Priyanshu-1477]

Hi! This is my first contribution to HTTPie — a small documentation improvement.
If any wording adjustments are preferred, I’m happy to update it. Thanks!