reflect,doc: use "the" instead of "a" in IsZero docs

There is a subtle distinction between a value
*being* the zero value vs being *equal to* the zero value.
This was discussed at length in #31450.

Using "a zero value" in the docs suggests that there may
be more than zero value. That is possible on the "equal to
zero value" reading, but not the "is zero" reading that we
selected for the semantics of IsZero.

This change attempts to prevent any confusion on this front by
switching to "the zero value" in the documentation.

And while we're here, eliminate a double-space.
(Darn macbook keyboards.)

Change-Id: Iaa02ba297438793f5a90be9919a4d53baef92f8e
Reviewed-on: https://go-review.googlesource.com/c/go/+/182617


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>