doc: readme-zephyr: fix scratch partition #1812

Job	Run time
environment (esp32, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secureb...	1m 53s
environment (esp32, serialrecovery)	1m 29s
environment (esp32s2, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	1m 43s
environment (esp32s2, serialrecovery)	1m 30s
environment (esp32s3, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	1m 45s
environment (esp32s3, serialrecovery)	1m 23s
environment (esp32c3, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	1m 46s
environment (esp32c3, serialrecovery)	1m 25s
environment (esp32, multi-image,multi-boot, multi)	1m 39s
environment (esp32s3, multi-image,multi-boot, multi)	1m 30s
	16m 3s

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