Fix a few typos.

testing/ostest/ostest_main.c

@@ -556,7 +556,7 @@ static int user_main(int argc, char *argv[])
       show_memory_usage(&g_mmbefore, &g_mmafter);
     }
 
-  printf("user_main: Exitting\n");
+  printf("user_main: Exiting\n");
   return 0;
 }
 

testing/ostest/rmutex.c

@@ -110,7 +110,7 @@ static FAR void *thread_outer(FAR void *parameter)
       thread_inner((int)((intptr_t)parameter), 0);
     }
 
-  printf("thread_outer[%d]: Exitting\n", (int)((intptr_t)parameter));
+  printf("thread_outer[%d]: Exiting\n", (int)((intptr_t)parameter));
   pthread_exit(NULL);
   return NULL; /* Non-reachable -- needed for some compilers */
 }

testing/ostest/robust.c

@@ -75,7 +75,7 @@ static FAR void *robust_waiter(FAR void *parameter)
     }
   else
     {
-      printf("robust_waiter: Exitting with mutex\n");
+      printf("robust_waiter: Exiting with mutex\n");
     }
 
   sleep(2);