encoding/json/v2: fix typo in comment

encoding/json/v2: fix typo in comment

Use the correct spelling of the Gregorian calendar.

Change-Id: I7e1d2974d38d5d3ded64f98852ee726c7b84be22
Reviewed-on: https://go-review.googlesource.com/c/go/+/704595
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Emmanuel Odeke <emmanuel@orijtech.com>