Adds aria label to custom picker on the demo page

[suryasu]

Pull request checklist

• Addresses an existing issue: Fixes Pickers A11y Pass #8655
• Include a change request file using $ yarn change

Description of changes

Adds aria label to custom picker on the demo page. This is not an issue with the custom picker component since the label should be defined by whoever creates the custom picker so that it's specific enough for screen readers

Microsoft Reviewers: Open in CodeFlow

[size-auditor]

Size Auditor did not detect a change in bundle size for any component!

[msft-github-bot]

Component Perf Analysis:

Scenario	Master Samples *	PR Samples *
BaseButton	701	681
BaseButton (experiments)	1559	1552
DefaultButton	932	933
DefaultButton (experiments)	1751	1772
DetailsRow	3995	4071
DetailsRow without styles	3996	4228
DocumentCardTitle with truncation	30599	30134
MenuButton	1769	1705
MenuButton (experiments)	4136	4171
PrimaryButton	1187	1127
PrimaryButton (experiments)	2046	2109
SplitButton	3291	3318
SplitButton (experiments)	7777	7791
Stack	442	440
Stack with Intrinsic children	1079	1070
Stack with Text children	4143	4075
Text	359	353
Toggle	814	832
Toggle (experiments)	2176	2172
button	64	58

* Sample counts can vary by up to 30% and shouldn't be used solely for determining regression. For more information please see the Perf Testing wiki.

[micahgodbolt]

Thanks for the PR! @joschect is the codeowner of this control. I'll wait for his sign off as he knows the control the best.

[msft-github-bot]

🎉[email protected] has been released which incorporates this pull request.:tada:

Handy links:

• Release Notes
• Official NPM Release
• Test the release with Developer Demo Site