Update PAYLOAD.md fixes #2449

[ramon-dejesus]

Creates PR in response to #2449

Description

I created a PR in reponse to @macdonst in issue #2449

Related Issue

#2449

Motivation and Context

This change provides information to Push Plugin developers that are having issues with iOS silent pushes not arriving in foreground.

How Has This Been Tested?

I tested this fix on iOS 12.1 and confirmed that it works.

Screenshots (if appropriate):

Types of changes

• [ X] 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 change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[macdonst]

@rldejesusm thanks for the PR

[andreszs]

Thank you for the PR. To show comments inside the JSON sample code properly, better use PHP format for syntax highlighting in this block. This way comments inside JSON will be displayed properly, with dark green foreground, because now they are shown like this:

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