encoding/json/jsontext: use consistent receiver

Other instances of encodeBuffer were using e as the receiver.
Switch to using e consistently.

Change-Id: Icb74c58ddde8aee7f2c6c16c4716034b0cf9ebd8
Reviewed-on: https://go-review.googlesource.com/c/go/+/775406
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: David Chase <drchase@google.com>