[NFC] Fix typos `occured` → `occurred` #6492

Job	Run time
spm-binary-cron	0s
changes	4s
check	0s
cmake-prod-db	0s
cmake	0s
pod-lib-lint	0s
pod-lib-lint-cron	0s
sanitizers-mac	0s
sanitizers-ubuntu	0s
spm-binary	0s
spm-source	0s
xcodebuild	0s
Check all required Firestore tests results	1s
	5s

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