text/template/parse: allow space after continue or break

Trivial fix: We must skip space after either of these keywords
before we expect a closing delimiter.

Also delete the stutter-generating extra 'in' in the error message.
(See what I did there?)

Fixes #51670

Change-Id: If5415632c36eaac6699bdc0aa6ce18be956c9b53
Reviewed-on: https://go-review.googlesource.com/c/go/+/392615


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Trust: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>