Update UI_CUSTOMIZATION.md

[rebeccamcginn]

colorContentLinkTextHover changes the link text color on hover

Problem

Currently states that colorContentLinkTextHover changes the link text color. It changes the link text color on hover.

Solution

Tiny change to state that colorContentLinkTextHover changes link text color on hover.

Checklist

put n/a if item is not relevant to PR changes

• Has the CHANGELOG been updated?
• Has the README been updated?
• Has the CONTRIBUTING doc been updated?
• Has the RELEASE_GUIDELINES been updated?
• Has the TESTING_GUIDELINES been updated?
• Has the MIGRATION doc been updated for any MAJOR breaking changes?
• Has the MIGRATION doc been updated for any MINOR breaking changes, including any translation strings or keys changes?
• Have any new automated tests been implemented or the existing ones changed?
• Have any new manual tests been written down or the existing ones changed?
• Have any new strings been translated or the existing ones changed?