StripePayments: fix 'occured' -> 'occurred' typos in STPBlocks doc comments

[SAY-5]

Four /// comments in StripePayments/StripePayments/Source/Helpers/STPBlocks.swift used error occured:

• line 32 — jsonResponse documentation
• line 93 — STPEphemeralKeyCompletionBlock error case
• line 95 — STPEphemeralKeyCompletionBlock unknown error case
• line 118 — error parameter documentation

Doc-only Swift change.

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[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}