fix vLLM capitalization

[aniketmaurya]

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

fix vLLM capitalization in README. The official name uses "v" in vLLM as lowercase letter.

PR review

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95%. Comparing base (c2d74cc) to head (3f5139f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #303   +/-   ##
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  Coverage    95%    95%           
===================================
  Files        19     19           
  Lines      1257   1257           
===================================
  Hits       1195   1195           
  Misses       62     62