[Dco] Remove preface in guide (#2096)

* fix

* fix

docs/source/guide/index.rst

@@ -5,7 +5,6 @@ User Guide
   :maxdepth: 2
   :titlesonly:
 
-  preface
   graph
   message
   nn

docs/source/index.rst

@@ -87,7 +87,6 @@ Getting Started
    :titlesonly:
    :glob:
 
-   guide/preface
    guide/graph
    guide/message
    guide/nn

python/dgl/heterograph.py

@@ -23,7 +23,7 @@ __all__ = ['DGLHeteroGraph', 'combine_names']
 class DGLHeteroGraph(object):
     """Class for storing graph structure and node/edge feature data.
 
-    There are a few ways to create create a DGLGraph:
+    There are a few ways to create a DGLGraph:
 
     * To create a homogeneous graph from Tensor data, use :func:`dgl.graph`.
     * To create a heterogeneous graph from Tensor data, use :func:`dgl.heterograph`.
@@ -1974,7 +1974,7 @@ class DGLHeteroGraph(object):
         return self.num_nodes(ntype)
 
     def num_nodes(self, ntype=None):
-        """Return the number of nodes of in the graph.
+        """Return the number of nodes in the graph.
 
         Parameters
         ----------