[TRIVIAL] Typo fix 'of' instead of 'on'

[hoos]

High Level Overview of Change

The class comment in AMMCreate.h used the word "on" instead of "of" in the sentence: "... AMM pool in the form on LPTokens."

Context of Change

A typo in comments.

Type of Change

• [ x] Documentation update

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[Bronek]

Please add [TRIVIAL] in this PR subject line.

[hoos]

I added [TRIVIAL] to the beginning of the subject line of the PR.

[intelliot]

It's truly a trivial change, but giving @gregtatcam - as the original author of AMM - a chance to see

[gregtatcam]

👍 LGTM

[intelliot]

Merged because this only touches a comment (no code).