testing: fix example for unexported identifier

Fixes golang#75540

Change-Id: I925f893d33660f0b08996d53cc9c564017232b39
Reviewed-on: https://go-review.googlesource.com/c/go/+/705456
Reviewed-by: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: seankhliao

src/testing/testing.go

@@ -30,9 +30,9 @@
 //	import "testing"
 //
 //	func TestAbs(t *testing.T) {
-//	    got := Abs(-1)
+//	    got := abs(-1)
 //	    if got != 1 {
-//	        t.Errorf("Abs(-1) = %d; want 1", got)
+//	        t.Errorf("abs(-1) = %d; want 1", got)
 //	    }
 //	}
 //