Fix spelling error in manual sam_overview 8

 - Changed usefull to useful

Change-Id: I2d7872b21e889202cd2b7752db4c76f18fffa95d
Signed-off-by: Richard B Winters <rik@mmogp.com>
Reviewed-by: Steven Dake <sdake@redhat.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>

man/sam_overview.8

@@ -160,7 +160,7 @@ Implicit way is achieved by setting recovery policy to QUIT and let process exit
 If this happened, object is not deleted and corosync watchdog will take required action.
 
 .P
-Explicit way is usefull for situations, when developer can deal with some non-fatal fall of application.
+Explicit way is useful for situations, when developer can deal with some non-fatal fall of application.
 This mode is achieved by setting policy to RESTART and using SAM same as without Confdb integration.
 If real fail is needed (like too many restarts at all, per/sec, ...), it's possible to use \fBsam_mark_failed(3)\fR
 and let corosync watchdog take required action.