Change code fence block language to json in markdown-reference.md

[ihor-kitsa]

Change code fence block language to json and add missing quotation mark in the Images section.

[learn-build-service-prod[bot]]

Learn Build status updates of commit 0961bda:

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[ihor-kitsa]

#sign-off

[ihor-kitsa]

#hold-off

[learn-build-service-prod[bot]]

Learn Build status updates of commit f76871c:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
Contribute/content/markdown-reference.md	:white_check_mark:Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[ihor-kitsa]

#sign-off