Change indentation.md to reflect no colon in extensions

Author: odersky

docs/docs/reference/other-new-features/indentation.md

@@ -59,6 +59,7 @@ There are two rules:
     An indentation region can start
 
      - after the condition of an `if-else`, or
+     - after the leading parameters of an `extension`, or
      - after a ": at end of line" token (see below)
      - after one of the following tokens:
     ```
@@ -120,7 +121,7 @@ enum Color:
 type T = A:
   def f: Int
 
-given [T] with Ord[T] as Ord[List[T]]:
+given [T](using Ord[T]) as Ord[List[T]]:
   def compare(x: List[T], y: List[T]) = ???
 
 extension (xs: List[Int])