Fix a typo in docs\examples\jms\derby-jdbc2-service.xml

There is a typo in JBOSS_HOME\docs\examples\jms\derby-jdbc2-service.xml that generates errors when using it to configure Derby as the default data source for the JMS system. The typo is in the PersistenceManager mbean definition:

<mbean code="org.jboss.mq.pm.jdbc2.PersistenceManager" name="jboss.mq:service=PersistenceManager">

Specifically in line 66 (in my editor at least) there is the string "EXITS" instead of the correct "EXISTS". Without this small change an SQL error is displayed in the server log:

14:34:02,233 WARN [ServiceController] Problem starting service jboss.mq:service=PersistenceManager org.jboss.mq.SpyJMSException: Could not resolve uncommited transactions. Message recovery may not be accurate; - nested throwable: (java.sql.SQLSyntaxErrorException: Syntax error: Encountered "SELECT" at line 1, column 39.)
at org.jboss.mq.pm.jdbc2.PersistenceManager.resolveAllUncommitedTXs(PersistenceManager.java:492)
at org.jboss.mq.pm.jdbc2.PersistenceManager.startService(PersistenceManager.java:1809)
at org.jboss.system.ServiceMBeanSupport.jbossInternalStart(ServiceMBeanSupport.java:289)
at org.jboss.system.ServiceMBeanSupport.jbossInternalLifecycle(ServiceMBeanSupport.java:245)
at sun.reflect.GeneratedMethodAccessor2.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.jboss.mx.interceptor.ReflectedDispatcher.invoke(ReflectedDispatcher.java:155)
at org.jboss.mx.server.Invocation.dispatch(Invocation.java:94)
at org.jboss.mx.server.Invocation.invoke(Invocation.java:86)
at org.jboss.mx.server.AbstractMBeanInvoker.invoke(AbstractMBeanInvoker.java:264)
at org.jboss.mx.server.MBeanServerImpl.invoke(MBeanServerImpl.java:659)
at org.jboss.system.ServiceController$ServiceProxy.invoke(ServiceController.java:978)
at $Proxy0.start(Unknown Source)
at org.jboss.system.ServiceController.start(ServiceController.java:417)
at org.jboss.system.ServiceController.start(ServiceController.java:435)
at org.jboss.system.ServiceController.start(ServiceController.java:435)
at org.jboss.system.ServiceController.start(ServiceController.java:435)
at org.jboss.system.ServiceController.start(ServiceController.java:435)
at sun.reflect.GeneratedMethodAccessor9.invoke(Unknown Source)
[...]