Update triggers-introduction.md

[gretchunkim]

error in trigger condition expression, using ?? instead of ? @equals(triggeroutputs()??['body/subject'], 'invoice approved'), is wrong

It should be @equals(triggeroutputs()?['body/subject'], 'invoice approved').

[prmerger-automator]

@gretchunkim : Thanks for your contribution! The author(s) have been notified to review your proposed change.

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[learn-build-service-prod]

Learn Build status updates of commit 1bbdecc:

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For more details, please refer to the build report.

For any questions, please:

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