doc: fix grammar and punctuation in buffer.md

[Rawal27]

Summary

Improve grammar and clarity in doc/api/buffer.md.

Changes

• Added the missing article “an” in the buf.fill() example.
• Added the missing article “a” in the multi-byte character example.
• Added “the” before offset in the buf.lastIndexOf() parameter description.
• Improved the buf.lastIndexOf() description by changing it to a complete sentence and adding “that” after “except”.
• Replaced “like” with “such as” when introducing examples of arguments that coerce to NaN.

No functional or behavioral changes are introduced; these are documentation-only improvements in grammar and clarity.

[aduh95]

Please stop opening PRs faster they can be reviewed. If you do not stop, you will get blocked from the organisation.

[Rawal27]

@aduh95 Can you clarify this? I mean, these are genuine doc-level changes that I have proposed. Also, a contributor can contribute to as many as 30 pull requests at a time. Closing them without reason seems unfair to me.

[Rawal27]

[Rawal27]