fix typo in comment (#870)

Author: WeihanLi

src/DotNetCore.CAP/Internal/TopicAttribute.cs

@@ -24,7 +24,7 @@ protected TopicAttribute(string name, bool isPartial = false)
         public string Name { get; }
 
         /// <summary>
-        /// Defines wether this attribute defines a topic subscription partial.
+        /// Defines whether this attribute defines a topic subscription partial.
         /// The defined topic will be combined with a topic subscription defined on class level,
         /// which results for example in subscription on "class.method".
         /// </summary>
@@ -37,4 +37,4 @@ protected TopicAttribute(string name, bool isPartial = false)
         /// </summary>
         public string Group { get; set; }
     }
-}
+}