Change the way of writing a extension method header

- This subsection explains generic extensions WITHOUT using clause(s) first,
  and then explains generic extensions WITH using clause(s).
    The changed code is for the first part, and don't introduce using clause(s)
  in it is more suitable in this context.

Author: ShapelessCat

docs/docs/reference/contextual/extension-methods.md

@@ -79,7 +79,7 @@ themselves:
 
 ```scala
 extension [T](xs: List[T])
-   def sumBy[U](f: T => U)(using Numeric[U]): U = ...
+   def sumBy[U: Numeric](f: T => U): U = ...
 ```
 
 Type arguments matching method type parameters are passed as usual: