Fix early binding example (#2847)

Fix early binding example.
Closes GH-2848.

docs/examples/userguide/early_binding_for_speed/rectangle_cdef.pyx

@@ -18,5 +18,5 @@ cdef class Rectangle:
         return self._area()
 
 def rectArea(x0, y0, x1, y1):
-    rect = Rectangle(x0, y0, x1, y1)
-    return rect.area()
+    cdef Rectangle rect = Rectangle(x0, y0, x1, y1)
+    return rect._area()

docs/examples/userguide/early_binding_for_speed/rectangle_cpdef.pyx

@@ -15,5 +15,5 @@ cdef class Rectangle:
         return area
 
 def rectArea(x0, y0, x1, y1):
-    rect = Rectangle(x0, y0, x1, y1)
+    cdef Rectangle rect = Rectangle(x0, y0, x1, y1)
     return rect.area()