Reword emoji autocomplete preference help text

What does this MR do and why?

Rewords the help text for the emoji autocomplete preference in user settings:

When you type a colon (:) in a description or comment box, a list of matching emoji is are suggested."

The old phrasing used "emoji" as plural but then said "a list of matching emoji is suggested", which reads awkwardly. The new phrasing is more natural and consistent with how GitLab uses "emoji" as an uncountable plural.

Closes Reword emoji autocomplete preference help text (#608288 - closed). Follow-up from Add preference to disable emoji autocomplete pi... (!246534 - merged).

How to set up and validate locally

1. Visit user preferences at /-/profile/preferences
2. Scroll to the "Behavior" section
3. Find the "Enable emoji autocomplete" checkbox
4. Verify the help text reads: "When you type a colon (:) in a description or comment box, matching emoji are suggested."

MR acceptance checklist

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