fix(guide): simplify directory structure

guide/english/csharp/linq/single-or-default/index.md

@@ -0,0 +1,28 @@
+---
+title: Single Or Default
+---
+
+# SingleOrDefault
+
+Returns the single element in a sequence that satisfies the specified condition or the default value if no element is found. The method will throw an exception if more than one element is found that satisfies the specified condition.
+
+### Signature
+```csharp
+Enumerable.SingleOrDefault<TSource>(IEnumerable<TSource>, Func<TSource, Boolean>)
+```
+
+## Example
+```csharp
+var fruits = new List<Fruit>() {
+    new Fruit() { Id = 1, Name = "Orange",     Color = "Orange", Quantity: 3   },
+    new Fruit() { Id = 2, Name = "Strawberry", Color = "Red",    Quantity: 12  },
+    new Fruit() { Id = 3, Name = "Grape",      Color = "Purple", Quantity: 25  },
+    new Fruit() { Id = 4, Name = "Pineapple",  Color = "Yellow", Quantity: 1   },
+    new Fruit() { Id = 5, Name = "Apple",      Color = "Red",    Quantity: 5   },
+    new Fruit() { Id = 6, Name = "Mango",      Color = "Yellow", Quantity: 2   }
+};
+
+var purpleFruit = fruits.SingleOrDefault(f => f.Color == "Purple"); // Grape
+
+var greenFruit = fruits.SingleOrDefault(f => f.Color == "Green"); // null
+```