[Doc] NN Modules Edit for grammar and style (#991)

* Edit for grammar and style

Improve the flow and readability

* Update docs/source/features/nn.rst

Better now? NN Modules as a title is vague

docs/source/features/nn.rst

 .. currentmodule:: dgl
 
-NN Modules
+Graph neural network modules
 ===============
 
-A set of high-level pre-defined modules are provided to build graph neural networks.
+This topic provides a link to several high-level, pre-defined modules you can use to build graph neural networks (NN).
 
 
 .. toctree::
@@ -13,11 +13,10 @@ A set of high-level pre-defined modules are provided to build graph neural netwo
    ../api/python/nn
 
 
-Contribution Guide
+Contribution guide
 ~~~~~~~~~~~~~~~~~~~~~
 
-We welcome your contribution! If you want a model to be implemented in DGL as a NN module,
-please `create an issue <https://github.com/dmlc/dgl/issues>`_ started with "[Feature Request] NN Module XXXModel".
+We welcome your contribution! If you want a model to be implemented in DGL as an NN module,
+please `create an issue <https://github.com/dmlc/dgl/issues>`_ and use this title style: "[Feature Request] NN Module XXXModel".
 
-If you want to contribute a NN module, please `create a pull request <https://github.com/dmlc/dgl/pulls>`_ started
-with "[NN] XXXModel in MXNet/PyTorch NN Modules" and our team member would review this PR.
+If you want to contribute an NN module, please `create a pull request <https://github.com/dmlc/dgl/pulls>`_ and use this title style: "[NN] XXXModel in MXNet/PyTorch NN Modules". A team member will review the pull request.