Fixes bad code sample for optional parameters in methods (#35528)

fixes #30383

Author: cartermp

docs/fsharp/language-reference/members/methods.md

@@ -98,9 +98,10 @@ Overloaded methods are methods that have identical names in a given type but tha
 Starting with F# 4.1, you can also have optional arguments with a default parameter value in methods.  This is to help facilitate interoperation with C# code.  The following example demonstrates the syntax:
 
 ```fsharp
+open System.Runtime.InteropServices
 // A class with a method M, which takes in an optional integer argument.
 type C() =
-    _.M([<Optional; DefaultParameterValue(12)>] i) = i + 1
+    member _.M([<Optional; DefaultParameterValue(12)>] i) = i + 1
 ```
 
 Note that the value passed in for `DefaultParameterValue` must match the input type.  In the above sample, it is an `int`.  Attempting to pass a non-integer value into `DefaultParameterValue` would result in a compile error.