[Doc] fix equation in nn.conv (#425)

python/dgl/nn/mxnet/conv.py

@@ -15,7 +15,7 @@ class GraphConv(gluon.Block):
     and can be described as below:
 
     .. math::
-      h_i^{(l+1)} = \sigma(b^{(l)} + \sum_{j\in\mathcal{N}(i)}\frac{1}{c_{ij}}W^{(l)}h_j^{(l)})
+      h_i^{(l+1)} = \sigma(b^{(l)} + \sum_{j\in\mathcal{N}(i)}\frac{1}{c_{ij}}h_j^{(l)}W^{(l)})
 
     where :math:`\mathcal{N}(i)` is the neighbor set of node :math:`i`. :math:`c_{ij}` is equal
     to the product of the square root of node degrees:

python/dgl/nn/pytorch/conv.py

@@ -16,7 +16,7 @@ class GraphConv(nn.Module):
     and can be described as below:
 
     .. math::
-      h_i^{(l+1)} = \sigma(b^{(l)} + \sum_{j\in\mathcal{N}(i)}\frac{1}{c_{ij}}W^{(l)}h_j^{(l)})
+      h_i^{(l+1)} = \sigma(b^{(l)} + \sum_{j\in\mathcal{N}(i)}\frac{1}{c_{ij}}h_j^{(l)}W^{(l)})
 
     where :math:`\mathcal{N}(i)` is the neighbor set of node :math:`i`. :math:`c_{ij}` is equal
     to the product of the square root of node degrees: