[SE-0346] Fix error in code snippet in Alternatives considered

The return type of a convert-to-`Double` function should be `Double`, not `Int`. This looks like a copy-paste oversight.

Author: ole

proposals/0346-light-weight-same-type-syntax.md

@@ -451,7 +451,7 @@ extension Convertible(from: String, to: Int) {
 }
 
 extension Convertible(from: String, to: Double) {
-  static func convert(_: String) -> Int
+  static func convert(_: String) -> Double
 }
 ```