fix indentation error

Author: airspeedswift

stdlib/public/core/Flatten.swift.gyb

@@ -416,10 +416,10 @@ extension LazyCollectionProtocol
   Elements : ${collectionForTraversal(traversal)},
 %   end
   ${constraints % {'Base': 'Elements.'}}
- {
-    /// A concatenation of the elements of `self`.
-    public func joined() -> LazyCollection<${Collection}<Elements>> {
-      return ${Collection}(elements).lazy
+{
+  /// A concatenation of the elements of `self`.
+  public func joined() -> LazyCollection<${Collection}<Elements>> {
+    return ${Collection}(elements).lazy
   }
 
   @available(*, unavailable, renamed: "joined()")