[release-branch.go1] html/template: fix URL doc

««« backport fead9e11a489
html/template: fix URL doc

This is the easy part of issue 3528.
(What to do about "noescape" is the hard part, left open.)

Update #3528.

R=mikesamuel, r, dsymonds
CC=golang-dev
https://golang.org/cl/6493113

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