change "Notes" for "Note" to keep visual effect in documentation

stix2/base.py

@@ -263,7 +263,7 @@ class _STIXBase(collections.Mapping):
         Returns:
             str: The serialized JSON object.
 
-        Notes:
+        Note:
             The argument ``pretty=True`` will output the STIX object following
             spec order. Using this argument greatly impacts object serialization
             performance. If your use case is centered across machine-to-machine

stix2/datastore/filesystem.py

@@ -102,7 +102,7 @@ class FileSystemSink(DataSink):
                 in a STIX object (or list of), dict (or list of), or a STIX 2.0
                 json encoded string.
 
-        Notes:
+        Note:
             ``stix_data`` can be a Bundle object, but each object in it will be
             saved separately; you will be able to retrieve any of the objects
             the Bundle contained, but not the Bundle itself.

stix2/datastore/taxii.py

@@ -304,7 +304,7 @@ class TAXIICollectionSource(DataSource):
         Does not put in TAXII spec format as the TAXII2Client (that we use)
         does this for us.
 
-        Notes:
+        Note:
             Currently, the TAXII2Client can handle TAXII filters where the
             filter value is list, as both a comma-seperated string or python
             list.