net/html: add missing package name in doc example
net/html: add missing package name in doc example

This code snippet should contains the prefix "html" like
other examples to be consistent.

Change-Id: I32428452625c016894aebc2011cde2dd614e6ed9
Reviewed-on: https://go-review.googlesource.com/77830
Reviewed-by: Gabriel Aszalos <08a2f807ca69c12efc020bd863a19890e920f1c4@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Gabriel Aszalos <08a2f807ca69c12efc020bd863a19890e920f1c4@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/net