Fix output size equations in Conv2D_cn.rst

Correct mathematical equations for output height and width calculations in Conv2D documentation.

Author: inc4dge

docs/api/paddle/nn/Conv2D_cn.rst

@@ -81,10 +81,10 @@ bias
     如果 ``padding`` = "SAME":
 
     .. math::
-        H_{out} = \frac{(H_{in} + stride[0] - 1)}{stride[0]}
+        H_{out} = \lceil \frac{H_{in}}{stride[0]} \rceil
 
     .. math::
-        W_{out} = \frac{(W_{in} + stride[1] - 1)}{stride[1]}
+        W_{out} = \lceil \frac{W_{in}}{stride[1]} \rceil
 
     如果 ``padding`` = "VALID":