testing/quick: simplify explanation of -quickchecks flag

CL 172698 documented this flag but the description was missing
punctuation and could be clearer.

Change-Id: I310d91ae8c6b947ce7d1ae7559882f49778f770a
Reviewed-on: https://go-review.googlesource.com/c/go/+/172817


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>