Fix Typographical Errors in Documentation and Code Comments

[Dimitrolito]

What does this pull request do?
This pull request corrects several typographical errors in documentation and code comments to improve clarity and adhere to grammatical standards.

Specific updates:

• Fixed incorrect usage of "a" vs. "an" in various files:
  • cmd/devtool/README.md: Updated "a orchestrator/transcoder" to "an orchestrator/transcoder."
  • cmd/livepeer_cli/wizard_transcoder.go: Corrected "a orchestrator" to "an orchestrator."
  • common/util.go: Changed "a fixed" to "an fixed."
  • doc/httpcli.md: Fixed "a HTTP" to "an HTTP."
  • doc/rtmpwebhookauth.md: Updated "a HTTP" to "an HTTP."
  • pm/stub.go: Corrected "a all" to "an all."

How did you test each of these updates?

• Manually reviewed all updated files to ensure correctness.
• Verified that changes do not affect functionality as they are limited to comments and documentation.

Does this pull request close any open issues?
No open issues are directly closed by this pull request.

Checklist:

• Read the contribution guide.
• make runs successfully.
• All tests in ./test.sh pass.
• Updated relevant documentation.
• Pending changelog updated, if applicable.

Notes

This PR ensures consistency and professionalism in project documentation and comments. No code functionality has been altered.

[thomshutt]

Thanks @Dimitrolito!