os: remove extraneous 's' from TestStdPipe error message

Change-Id: I73674a90d88da630b09dcb75ced5d9f0d96c3c09
Reviewed-on: https://go-review.googlesource.com/43153
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/pipe_test.go

@@ -87,7 +87,7 @@ func TestStdPipe(t *testing.T) {
 					t.Errorf("unexpected SIGPIPE signal for descriptor %d sig %t", dest, sig)
 				}
 			} else {
-				t.Errorf("unexpected exit status %v for descriptor %ds sig %t", err, dest, sig)
+				t.Errorf("unexpected exit status %v for descriptor %d sig %t", err, dest, sig)
 			}
 		}
 	}