docs: fix CNTRL_2 DATA_FORMAT field

[spectrum70]

Noted that in my application, ad3552r-axi, i need to set this bit (4) to 0 to have unsigned 16bit samples generated.

PR Description

Fix DAC IP documentation.

PR Type

• [x] Bug fix (change that fixes an issue)
• [ ] New feature (change that adds new functionality)
• [ ] Breaking change (has dependencies in other repos or will cause CI to fail)

PR Checklist

• [ ] I have followed the code style guidelines
• [x] I have performed a self-review of changes
• [ ] I have compiled all hdl projects and libraries affected by this PR
• [ ] I have tested in hardware affected projects, at least on relevant boards
• [ ] I have commented my code, at least hard-to-understand parts
• [x] I have signed off all commits from this PR
• [ ] I have updated the documentation (wiki pages, ReadMe files, Copyright etc)
• [ ] I have not introduced new Warnings/Critical Warnings on compilation
• [ ] I have added new hdl testbenches or updated existing ones

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[sarpadi]

Tagging @caosjr