MNT Update selector in behat to reflect changes in silverstripe/admin #458

Job	Run time
CI / Context	1s
CI / Generate matrix	19s
CI / Check governance	0s
CI / 8.3 prf-low mariadb phpunit all	1m 56s
CI / 8.3 mysql80 phpunit all	1m 58s
CI / 8.3 mysql84 phpunit all	1m 50s
CI / 8.3 mysql80 phplinting	1m 45s
CI / 8.3 mysql80 endtoend root	11m 2s
CI / 8.3 mysql80 js	3m 59s
CI / Dispatch tag patch release	0s
	22m 50s

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