Modify doc issues about property option of aggregate

The formal of multi properties in most docs likes this:
[--property <key=value> [...] ]
[--property <key> [...] ]
This patch modify the formal of multi properties in
aggregate.rst.

Change-Id: I557cd51696d2721c8955c4a79f1ab85de3a0b9ce

Author: Huanxuan Ao

doc/source/command-objects/aggregate.rst

@@ -38,7 +38,7 @@ Create a new aggregate
 
     os aggregate create
         [--zone <availability-zone>]
-        [--property <key=value>]
+        [--property <key=value> [...] ]
         <name>
 
 .. option:: --zone <availability-zone>
@@ -115,7 +115,7 @@ Set aggregate properties
     os aggregate set
         [--name <new-name>]
         [--zone <availability-zone>]
-        [--property <key=value>]
+        [--property <key=value> [...] ]
         <aggregate>
 
 .. option:: --name <name>
@@ -160,8 +160,7 @@ Unset aggregate properties
 .. code-block:: bash
 
     os aggregate unset
-        --property <key>
-        [--property <key>] ...
+        [--property <key> [...] ]
         <aggregate>
 
 .. option:: --property <key>