Fix syntax of json #15018

Job	Run time
Core Basic (18.x)	6m 36s
Core Basic (20.x)	6m 27s
Core Basic - Plone 5 (18.x)	6m 42s
Core Blocks (18.x)	8m 5s
Core Blocks - Listing (18.x)	4m 45s
Core Blocks (20.x)	8m 17s
Core Blocks - Listing (20.x)	4m 50s
Core Content (18.x)	3m 37s
Core Content (20.x)	3m 33s
Core Control Panels (18.x)	4m 19s
Coresandbox (18.x)	4m 50s
Core Control Panels (20.x)	4m 29s
Core Volto Slate (18.x)	7m 34s
Core Volto Slate (20.x)	7m 56s
Project Generator (18.x)	7m 4s
Multilingual (18.x)	3m 42s
Multilingual in Seamless Mode (18.x)	3m 55s
Seamless Mode (18.x)	6m 58s
Project Generator (20.x)	7m 30s
Working Copy (18.x)	3m 30s
	1h 54m 39s

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