fix incorrect docstring for dispatcher

The option name is dispatcher, but the examples in docstrings of
dispatcher plugins name it as dispatchers, which is incorrect.

Change-Id: I13dd5af4a137e34e73958458125dd5d3363c49ec

ceilometer/dispatcher/database.py

@@ -32,7 +32,8 @@ class DatabaseDispatcher(dispatcher.Base):
     To enable this dispatcher, the following section needs to be present in
     ceilometer.conf file
 
-    dispatchers = database
+    [DEFAULT]
+    dispatcher = database
     """
     def __init__(self, conf):
         super(DatabaseDispatcher, self).__init__(conf)

ceilometer/dispatcher/file.py

@@ -48,8 +48,8 @@ class FileDispatcher(dispatcher.Base):
     To enable this dispatcher, the following section needs to be present in
     ceilometer.conf file
 
-    [collector]
-    dispatchers = file
+    [DEFAULT]
+    dispatcher = file
     """
 
     def __init__(self, conf):

ceilometer/dispatcher/http.py

@@ -53,7 +53,8 @@ class HttpDispatcher(dispatcher.Base):
     To enable this dispatcher, the following option needs to be present in
     ceilometer.conf file::
 
-        dispatchers = http
+        [DEFAULT]
+        dispatcher = http
 
     Dispatcher specific options can be added as follows::