testing/fstest: clarify TestFS docs

The sentence starts "fsys must only contain",
which leads the reader to believe that fsys must not contain others.
The rapid reversal leads to confusion.
I had to read it several times to be sure I'd parsed it correctly.

Remove "only"; rely on the rest of the sentence to clarify.

Change-Id: I9fb7935aed4f9839344d3a00b761d20981fba864
Reviewed-on: https://go-review.googlesource.com/c/go/+/296529


Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>