Change long term to long-term in readme

[danalamo]

Changes in this pull request

Issue fixed: #760

Checklist

• All tests pass. Demo project builds and runs.
• I added tests, an experiment, or detailed why my change isn't tested.
• I added an entry to the CHANGELOG.md for any breaking changes, enhancements, or bug fixes.
• I have reviewed the contributing guide

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[jessesquires]

Thanks @danalamo !

Looks like this change is included in #780. Let's close this one.