docs(proxy-events.md): fix example

[pstephan9]

The notation {'a', b} isn't syntactically correct nowadays, preferring {a: b}

Description

Motivation and Context

How has this been tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[coveralls]

coverage: 97.619%. remained the same when pulling 05a553a on pstephan9:patch-1 into cd58f96 on chimurai:master.

[chimurai]

Thanks!