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/+/172817Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/testing/quick/quick.go

@@ -180,8 +180,8 @@ type Config struct {
 	MaxCount int
 	// MaxCountScale is a non-negative scale factor applied to the
 	// default maximum.
-	// If zero, the default is configured by the -quickchecks flag
-	// which defaults to 100.
+	// A count of zero implies the default, which is usually 100
+	// but can be set by the -quickchecks flag.
 	MaxCountScale float64
 	// Rand specifies a source of random numbers.
 	// If nil, a default pseudo-random source will be used.