testing: fix wording in explanation of potential bug

TBR=rsc

R=rsc
CC=golang-dev
https://golang.org/cl/5504075

src/pkg/testing/testing.go

@@ -240,9 +240,9 @@ func RunTests(matchString func(pat, str string) (bool, error), tests []InternalT
 		runtime.GOMAXPROCS(procs)
 		// We build a new channel tree for each run of the loop.
 		// collector merges in one channel all the upstream signals from parallel tests.
-		// If all tests pump to the same channel, a bug can occur where a goroutine
-		// kicks off a test, fails, and still delivers a completion signal, which skews the
-		// counting.
+		// If all tests pump to the same channel, a bug can occur where a test
+		// kicks off a gouroutine that Fails, yet the test still delivers a completion signal,
+		// which skews the counting.
 		var collector = make(chan interface{})
 
 		numParallel := 0