Fix format display error

python/paddle/fluid/layers/nn.py

@@ -1180,12 +1180,12 @@ def conv2d(input,
 
         - Input:
 
-          Input shape: $(N, C_{in}, H_{in}, W_{in})$
+          Input shape: :math:`(N, C_{in}, H_{in}, W_{in})`
 
-          Filter shape: $(C_{out}, C_{in}, H_f, W_f)$
+          Filter shape: :math:`(C_{out}, C_{in}, H_f, W_f)`
 
         - Output:
-          Output shape: $(N, C_{out}, H_{out}, W_{out})$
+          Output shape: :math:`(N, C_{out}, H_{out}, W_{out})`
 
         Where