Add `else if`

tutorials/shaders/shader_reference/shading_language.rst

@@ -551,9 +551,11 @@ Godot Shading language supports the most common types of flow control:
 
 .. code-block:: glsl
 
-    // `if` and `else`.
+    // `if`, `else if` and `else`.
     if (cond) {
 
+    } else if (cond) {
+
     } else {
 
     }