fix links to native protocol docs

Author: Bulat Shakirzyanov

ext/doc/Cassandra/Exception/ReadTimeoutException.php

@@ -21,6 +21,6 @@
 /**
  * ReadTimeoutException is raised when a coordinator failed to receive acks
  * from the required number of replica nodes in time during a read.
- * @see https://github.com/apache/cassandra/blob/trunk/doc/native_protocol_v1.spec#L709-L726 Description of ReadTimeout error in the native protocol spec
+ * @see https://github.com/apache/cassandra/blob/cassandra-2.1/doc/native_protocol_v1.spec#L709-L726 Description of ReadTimeout error in the native protocol spec
  */
 class ReadTimeoutException extends ExecutionException {}

ext/doc/Cassandra/Exception/UnavailableException.php

@@ -23,6 +23,6 @@
  * enough replica nodes available to fulfill the request.
  *
  * NOTE: Request has not even been forwarded to the replica nodes in this case.
- * @see https://github.com/apache/cassandra/blob/trunk/doc/native_protocol_v1.spec#L667-L677 Description of the Unavailable error in the native protocol v1 spec.
+ * @see https://github.com/apache/cassandra/blob/cassandra-2.1/doc/native_protocol_v1.spec#L667-L677 Description of the Unavailable error in the native protocol v1 spec.
  */
 class UnavailableException extends ExecutionException {}

ext/doc/Cassandra/Exception/WriteTimeoutException.php

@@ -21,6 +21,6 @@
 /**
  * WriteTimeoutException is raised when a coordinator failed to receive acks
  * from the required number of replica nodes in time during a write.
- * @see https://github.com/apache/cassandra/blob/trunk/doc/native_protocol_v1.spec#L683-L708 Description of WriteTimeout error in the native protocol spec
+ * @see https://github.com/apache/cassandra/blob/cassandra-2.1/doc/native_protocol_v1.spec#L683-L708 Description of WriteTimeout error in the native protocol spec
  */
 class WriteTimeoutException extends ExecutionException {}