[skip changelog] Replace gender-specific grammar in package_index.json specification

[per1234]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests before creating one)
• The PR follows our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce?

Docs update.

• What is the current behavior?

package_index.json specification contains gender-specific grammar.

• What is the new behavior?

package_index.json specification uses singular "their" in place of gender-specific grammar.

• Does this PR introduce a breaking change?

No.

• Other information:

This violates traditional rules of grammar. I don't think that's a problem, but there is the altenative of rewording the sentence to dodge the issue entirely:

Many different index files coming from different vendors may be in use, so vendors should name their own index files in a way that won't conflict with others.

However, I feel this makes the meaning of the sentence slightly less clear.