comment fix

improve English a bit. :-)

sentinel.conf

@@ -63,7 +63,7 @@ sentinel failover-timeout mymaster 900000
 # The script is called with just two arguments: the first is the event type
 # and the second the event description.
 #
-# The script must be exits and executable in order for sentinel to start if
+# The script must exist and be executable in order for sentinel to start if
 # this option is provided.
 #
 # Example: