regexp: clarify example for 'FindString'

Clarifying that FindString only provides left-most match

Change-Id: Ic6ecec12cca759fd4b3565ef5901a110843ffd56
Reviewed-on: https://go-review.googlesource.com/48609Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/regexp/example_test.go

@@ -39,11 +39,11 @@ func ExampleMatchString() {
 }
 
 func ExampleRegexp_FindString() {
-	re := regexp.MustCompile("fo.?")
-	fmt.Printf("%q\n", re.FindString("seafood"))
+	re := regexp.MustCompile("foo.?")
+	fmt.Printf("%q\n", re.FindString("seafood fool"))
 	fmt.Printf("%q\n", re.FindString("meat"))
 	// Output:
-	// "foo"
+	// "food"
 	// ""
 }