Updating MTD conv radius/thresh description

Pull Request Testing

[x] Describe testing already performed for these changes:
Checked to see that they passed and that documentation looked tood
[x] Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:
Check that there are no more references to MTD_FCST/OBS_CONV_RADIUS and MTD_FCST/OBS_CONV_THRESH. Also the MTD conv radius and threshold should all say they need a single number (rather than a list) https://metplus.readthedocs.io/en/bugfix_mtd_documentation/
[x] Do these changes include sufficient documentation updates, ensuring that no errors or warnings exist in the build of the documentation? [Yes or No] yes
[x] Do these changes include sufficient testing updates? [Yes or No] No sure??? It was a simple documentation fix
[x] Will this PR result in changes to the test suite? [Yes or No]
If yes, describe the new output and/or changes to the existing output:
No
[x] Do these changes introduce new SonarQube findings? [Yes or No]
If yes, please describe: No
[ ] Please complete this pull request review by [Fill in date].

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