Fix documentation typos in `ofxClipPreferences`

# Open Effects Proposal for Standard Change

Please read the contribution [guidelines](https://github.com/ofxa/openfx/wiki/Extending-OpenFX-Guidelines#submit-a-proposal-or-bug-report-to-review) first.

## Standard Change Workflow
- [x] Create proposal as issue (you're doing this now!)
- [x] Tag this issue with `standard change` tag
- [x] Identify subcommittee: at least one plug-in vendor, and at least one host
- [x] Discuss the idea in this issue
- [x] Write new or updated code and doc
- [x] Publish updates as a pull request (ideally on a `feature/PROPOSAL-NAME` branch)
- [x] Make sure that PR references this issue number to keep them in sync
- [x] Discuss and review code in the PR
- [ ] Meet all requirements below for accepting PR
- [x] When subcommittee signs off and other members don't have any further review comments,
maintainer merges PR to master which closes PR and issue

## Requirements for accepting a standard change:
- [x] Header files updated
- [x] Documentation updated
- [x] Release notes added
- [x] Compatibility review completed
- [x] Working code demonstrated with at least one host and one plugin
- [x] At least two members sign off
- [x] No further changes requested from membership

## Summary

Fix some doc typos is `ofxClipPreferences.rst`

## Motivation

As a professional standard, doc should be free of typos.

## Problem

Several typos exist on this page.

## Impact

Doc only, no code change.

## Documentation Impact

Docs should be rebuilt to test that this is done correctly.

## Stakeholders

All

## Discussion

Start with ofxClipPreferences.rst and review the documented text for the typos described in the issue. Correct the documentation only, then rebuild the docs to verify the changes; done means the typos are fixed and the documentation builds successfully.

Written by the indexing model from the issue text.