[otelcol docs] Copy edit the Building a custom Collector page

[tiffany76]

What needs to be changed?

Copy edit the Building a custom Collector page.

What is the name + path of the page that needs changed?

https://opentelemetry.io/docs/collector/custom-collector/, but this path will change if #8360 is complete.

Additional context:

Conduct a general copy edit, looking for typos, missing words, and bad formatting. Also edit the section for the following things:

• Style guide compliance
  • Use sentence case subheadings. Only proper nouns should be capitalized after the first word of the heading.
  • Capitalize OpenTelemetry proper nouns and de-capitalize general nouns (for example, Collector is capitalized when referring to the OTel Collector, but signal names such as span or trace are not capitalized.
  • Use “for example” instead of e.g. and “in other words” (or similar) for i.e.
  • Substitute “using”, “with”, “by”, etc. for "via"
  • Use short code admonitions instead of Markdown notes
• Clarity - rewrite if necessary
• Grammar
  • Unnecessary passive voice
  • Unnecessary adverbs and general wordiness
  • Future tense - should almost always be present tense
• Tone/voice continuity - look for any obvious shifts in tone or voice and correct them
• Request a review for accuracy from the Collector SIG.

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