Fix Typo in PluginBaseClassNotFoundException

[mkreusch]

1. Why is this change necessary?

The message is displayed to shop admins. We should have correct grammar/spelling here.

2. What does this change do, exactly?

Only fix a simple spelling mistake.

3. Describe each step to reproduce the issue or behaviour.

4. Please link to the relevant issues (if any).

5. Checklist

• I have rebased my changes to remove merge conflicts
• I have written tests and verified that they fail without my change
• I have created a changelog file with all necessary information about my changes
• I have written or adjusted the documentation according to my changes
• This change has comments for package types, values, functions, and non-obvious lines of code
• I have read the contribution requirements and fulfil them.

[CLAassistant]

All committers have signed the CLA.

[github-actions]

	Warnings
⚠️	The Pull Request doesn't contain any changelog file

[shopware-github-importer]

Hello,

thank you for creating this pull request.
I have opened an issue on our Issue Tracker for you. See the issue link: https://issues.shopware.com/issues/NEXT-35577

Please use this issue to track the state of your pull request.