Add spaces before and after closure arrow in stdlib

stdlib/public/core/StringCharacterView.swift

@@ -46,7 +46,7 @@ extension String {
   ///   that is the target of this method) during the execution of
   ///   `body`: it may not appear to have its correct value.  Instead,
   ///   use only the `String.CharacterView` argument to `body`.
-  public mutating func withMutableCharacters<R>(body: (inout CharacterView)->R) -> R {
+  public mutating func withMutableCharacters<R>(body: (inout CharacterView) -> R) -> R {
     // Naively mutating self.characters forces multiple references to
     // exist at the point of mutation. Instead, temporarily move the
     // core of this string into a CharacterView.