spec: add comment marker for consistency.

LGTM=r
R=gri, r
CC=golang-codereviews
https://golang.org/cl/185830043

doc/go_spec.html

@@ -5579,7 +5579,7 @@ <h3 id="Appending_and_copying_slices">Appending to and copying slices</h3>
 s4 := append(s3[3:6], s3[2:]...)   // append overlapping slice    s4 == []int{3, 5, 7, 2, 3, 5, 7, 0, 0}
 
 var t []interface{}
-t = append(t, 42, 3.1415, "foo")                                  t == []interface{}{42, 3.1415, "foo"}
+t = append(t, 42, 3.1415, "foo")   //                             t == []interface{}{42, 3.1415, "foo"}
 
 var b []byte
 b = append(b, "bar"...)            // append string contents      b == []byte{'b', 'a', 'r' }