Minor: fix sasl.kerberos.service.name

sasl.kerberos.service.name surround by double quote didn't work, have to remove.

Author: BINLEI XUE <kongpo0412@gmail.com>

Reviewers: Gwen Shapira

Closes #720 from secjex/patch-1

docs/security.html

@@ -231,7 +231,7 @@ Apache Kafka allows clients to connect over SSL. By default SSL is disabled but
 
         We must also configure the service name in server.properties, which should match the principal name of the kafka brokers. In the above example, principal is "kafka/kafka1.hostname.com@EXAMPLE.com", so:
         <pre>
-    sasl.kerberos.service.name="kafka"</pre>
+    sasl.kerberos.service.name=kafka</pre>
 
         <u>Important notes:</u>
         <ol>
@@ -270,7 +270,7 @@ Apache Kafka allows clients to connect over SSL. By default SSL is disabled but
             <li>Configure the following properties in producer.properties or consumer.properties:
                 <pre>
     security.protocol=SASL_PLAINTEXT (or SASL_SSL)
-    sasl.kerberos.service.name="kafka"</pre>
+    sasl.kerberos.service.name=kafka</pre>
             </li>
         </ol></li>
 </ol>