[GraphBolt] Fix GPUCachedFeature docstring typo (dmlc#7639)

Author: pyynb

python/dgl/graphbolt/impl/gpu_cached_feature.py

@@ -89,7 +89,7 @@ def read(self, ids: torch.Tensor = None):
         return values
 
     def read_async(self, ids: torch.Tensor):
-        """Read the feature by index asynchronously.
+        r"""Read the feature by index asynchronously.
 
         Parameters
         ----------
@@ -104,7 +104,7 @@ def read_async(self, ids: torch.Tensor):
             can be accessed by calling `.wait()`. on the returned future object.
             It is undefined behavior to call `.wait()` more than once.
 
-        Example Usage
+        Examples
         --------
         >>> import dgl.graphbolt as gb
         >>> feature = gb.Feature(...)