Fix typo: occured -> occurred in namespace_storage_app.vue

Mridankan Mandalrequested to merge
gitlab-community/gitlab-org/gitlab:fix/namespace-storage-app-occurred into master

What does this MR do and why?

Corrects a spelling mistake in a user-facing string in app/assets/javascripts/usage_quotas/storage/namespace/components/namespace_storage_app.vue.

Changes occured to occurred.

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