Fix E -> Ea in creating-mechanisms.md

[rwest]

Changes proposed in this pull request

The instructions on writing YAML files said to use E for the activation energy, but all (but one of) the examples use Ea, which I think is correct. I updated the instructions and the one example that had E.

If applicable, provide an example illustrating new features this pull request is introducing

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review

[ischoegl]

Thanks - looks good to me!