Replace string with str in rtypes.

Used the command:
ag -l 'rtype: string' | xargs sed -i .bak 's/rtype: string/rtype: str/g'

Based on this comment:
googleapis/google-cloud-python#2485 (comment)

`str` is a type, `string` is a module.

google/cloud/dns/changes.py

@@ -74,7 +74,7 @@ def _set_properties(self, resource):
  def path(self):
  """URL path for change set APIs.
 
- :rtype: string
+ :rtype: str
  :returns: the path based on project, zone, and change set names.
  """
  return '/projects/%s/managedZones/%s/changes/%s' % (
@@ -84,7 +84,7 @@ def path(self):
  def name(self):
  """Name of the change set.
 
- :rtype: string or ``NoneType``
+ :rtype: str or ``NoneType``
  :returns: Name, as set by the back-end, or None.
  """
  return self._properties.get('id')
@@ -104,7 +104,7 @@ def name(self, value):
  def status(self):
  """Status of the change set.
 
- :rtype: string or ``NoneType``
+ :rtype: str or ``NoneType``
  :returns: Status, as set by the back-end, or None.
  """
  return self._properties.get('status')

google/cloud/dns/zone.py

@@ -81,7 +81,7 @@ def from_api_repr(cls, resource, client):
  def project(self):
  """Project bound to the zone.
 
- :rtype: string
+ :rtype: str
  :returns: the project (derived from the client).
  """
  return self._client.project
@@ -90,7 +90,7 @@ def project(self):
  def path(self):
  """URL path for the zone's APIs.
 
- :rtype: string
+ :rtype: str
  :returns: the path based on project and dataste name.
  """
  return '/projects/%s/managedZones/%s' % (self.project, self.name)
@@ -117,7 +117,7 @@ def name_servers(self):
  def zone_id(self):
  """ID for the zone resource.
 
- :rtype: string, or ``NoneType``
+ :rtype: str, or ``NoneType``
  :returns: the ID (None until set from the server).
  """
  return self._properties.get('id')
@@ -126,7 +126,7 @@ def zone_id(self):
  def description(self):
  """Description of the zone.
 
- :rtype: string, or ``NoneType``
+ :rtype: str, or ``NoneType``
  :returns: The description as set by the user, or None (the default).
  """
  return self._properties.get('description')
@@ -153,7 +153,7 @@ def name_server_set(self):
  See:
  https://cloud.google.com/dns/api/v1/managedZones#nameServerSet
 
- :rtype: string, or ``NoneType``
+ :rtype: str, or ``NoneType``
  :returns: The name as set by the user, or None (the default).
  """
  return self._properties.get('nameServerSet')