Update RichSuggestBox.md

[deltoro05]

This URL https://unicode-table.com/ has changed to https://symbl.cc/

[learn-build-service-prod[bot]]

Learn Build status updates of commit a7679d4:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
docs/controls/RichSuggestBox.md	:white_check_mark:Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel