draft-terminology: add reference for pronouns text

[jlivingood]

Draft says:

• Use the neutral "they" as the singular pronoun and

Suggest adding a reference to the RFC Style Guide at https://www.rfc-editor.org/styleguide/part2/. This has a section on gender-specific language:

"Gender-Specific Language

Avoid using gender-specific language (e.g., “his” and “her”) when possible. Consider whether gender-neutral terms could be used instead (e.g., “the user”, “its”, and “their”)."

Suggested draft update:

• Use the neutral "they" as the singular pronoun and