[w3c/push-api] update reference to FIPS-186-4 (#210)

[FIPS-186-3] on the reference list has been superseded by FIPS-186-4 published in July 2013.  [WEBPUSH-ENCRYPTION] references FIPS-186-4 already.  It'd be nice if the spec could be updated to keep consistency with [WEBPUSH-ENCRYPTION].  BTW, there is no technical difference in terms of how P-256 should work.

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