MNT Update selector in behat to reflect changes in silverstripe/admin #455

Job	Run time
CI / Context	1s
CI / Generate matrix	13s
CI / Check governance	0s
CI / 8.3 prf-low mariadb phpunit all	2m 17s
CI / 8.3 mysql80 phpunit all	2m 16s
CI / 8.3 mysql84 phpunit all	2m 13s
CI / 8.3 mysql80 phplinting	2m 6s
CI / 8.3 mysql80 endtoend root	10m 54s
CI / 8.3 mysql80 js	4m 38s
CI / Dispatch tag patch release	0s
	24m 38s

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