fix doc (#2604)

docs/source/api/python/nn.rst

@@ -26,4 +26,4 @@ GNN-like structures, e.g.
 .. autosummary::
     :toctree: ../../generated/
 
-   nn.functional.edge_softmax
+   functional.edge_softmax

python/dgl/ops/edge_softmax.py

@@ -7,11 +7,11 @@ __all__ = ['edge_softmax']
 
 
 def edge_softmax(graph, logits, eids=ALL, norm_by='dst'):
-    r"""
+    r"""Compute softmax over weights of incoming edges for every node.
 
     Description
     -----------
-    Compute edge softmax. For a node :math:`i`, edge softmax is an operation that computes
+    For a node :math:`i`, edge softmax is an operation that computes
 
     .. math::
       a_{ij} = \frac{\exp(z_{ij})}{\sum_{j\in\mathcal{N}(i)}\exp(z_{ij})}