Fixed typo.

src/memtest.c

@@ -129,7 +129,7 @@ void memtest(size_t megabytes, int passes) {
     }
     memtest_test(megabytes,passes);
     printf("\nYour memory passed this test.\n");
-    printf("Please if you are stil in doubt use the following two tools:\n");
+    printf("Please if you are still in doubt use the following two tools:\n");
     printf("1) memtest86: http://www.memtest86.com/\n");
     printf("2) memtester: http://pyropus.ca/software/memtester/\n");
     exit(0);