Fix comment in "PRBMath.sqrt"

[PaulRBerg]

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As @nonergodic has aptly mentioned here, the comment on line 604 of PRBMath.sol is incorrect:

https://github.com/paulrberg/prb-math/blob/e33a042e4d1673fe9b333830b75c4765ccf3f5f2/contracts/PRBMath.sol#L604

Instead the comment should say something along the line of:

Set the initial guess to the largest power of two that is smaller than or equal to sqrt(x)

Relevant link: see OpenZeppelin's implementation.