Fix article usage in documentation and code comments

[Dimitrolito]

This pull request addresses typos and grammar issues related to the incorrect usage of articles ("a" vs "an") in multiple files. The changes involve correcting the articles to improve clarity and adherence to grammatical standards:

• Replaced "an" with "a" in phrases where the following word doesn't start with a vowel sound (e.g., "an low-degree oracle" → "a low-degree oracle").
• Replaced "a" with "an" in cases where the following word starts with a vowel sound (e.g., "a improper subset" → "an improper subset").
• Updated comments and documentation across various Rust files to reflect these grammatical corrections.

Files modified:

• README.md
• ark-bcs/src/bcs/constraints/verifier.rs
• ark-bcs/src/bcs/verifier.rs
• ark-bcs/src/iop/message.rs
• ark-bcs/src/iop/mod.rs
• ark-bcs/src/ldt/constraints/rl_ldt.rs
• ark-bcs/src/ldt/mod.rs

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Checklist:

• Targeted PR against correct branch (master)
• Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer