docs: update type redeclaration example

doc/docs.md

@@ -653,13 +653,18 @@ import time
 
 type MyTime time.Time
 
+fn (mut t MyTime) century() int {
+    return 1 + t.year % 100
+}
+
 fn main() {
     my_time := MyTime{
         year: 2020,
         month: 12,
         day: 25
     }
-    println(my_time.unix_time())
+    println(time.new_time(my_time).utc_string())
+    println('Century: ${my_time.century()}')
 }
 ```