docs: fix vanilla api examples

[dddieon]

style={{
  ['--cssVariableName']: 'red', // correct
  ['var(--cssVariableName)']: 'red', // wrong
}}

Because the variable named colorVar has the value var(--cssVariableName), css variable was not added as in the example.

To use colorVar directly in the inline style prop, delete 'var(',')' from the colorVar string, or use it like this commit.

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[Mokshit06]

Thanks for the fix!