encoding: fix a few function names on comments

encoding: fix a few function names on comments

Change-Id: I17a311afb94a056b3d35bfa241f5d0d206db602d
GitHub-Last-Rev: 42129464c9e17fae9b61ea60940e193fcefc5760
GitHub-Pull-Request: golang/go#55962
Reviewed-on: https://go-review.googlesource.com/c/go/+/436882
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Rob Pike <r@golang.org>