Fix guide example callback name mismatch

The cursor position callback is named differently in example usage and
example declaration.

Closes #1364.

docs/input.dox

@@ -245,7 +245,7 @@ If you wish to be notified when the cursor moves over the window, set a cursor
 position callback.
 
 @code
-glfwSetCursorPosCallback(window, cursor_pos_callback);
+glfwSetCursorPosCallback(window, cursor_position_callback);
 @endcode
 
 The callback functions receives the cursor position, measured in screen