docs: Fix sample code for folder locks endpoint

[kojimaru14]

Replace client.folders with client.folder

When you run the original sample code, you'll get this error:

AttributeError: 'Client' object has no attribute 'folders'. Did you mean: 'folder'?

[CLAassistant]

All committers have signed the CLA.

[coveralls]

Pull Request Test Coverage Report for Build 13800750135

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.607%

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Totals
Change from base Build 12746504069:	0.0%
Covered Lines:	3529
Relevant Lines:	3770

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💛 - Coveralls

[mwwoda]

Thanks for the submission