Fixed outdated comment

assert/assertions.go

@@ -1327,7 +1327,7 @@ func typeAndKind(v interface{}) (reflect.Type, reflect.Kind) {
 }
 
 // diff returns a diff of both values as long as both are of the same type and
-// are a struct, map, slice or array. Otherwise it returns an empty string.
+// are a struct, map, slice, array or string. Otherwise it returns an empty string.
 func diff(expected interface{}, actual interface{}) string {
 	if expected == nil || actual == nil {
 		return ""