chapters/io: Fix typo in mmap_cp task

Correct the name of wrapper from `ftstat()` to `fstat()`.

Signed-off-by: Andrei Adrian Ragman <[email protected]>

chapters/io/file-descriptors/drills/tasks/mmap_cp/solution/src/mmap_cp.c

@@ -45,7 +45,7 @@ int main(int argc, char *argv[])
 	 * of the output file to the size of the input file.
 	 */
 
-	/* TODO 2: Use ftstat() to get the size of the input file. */
+	/* TODO 2: Use fstat() to get the size of the input file. */
 	rc = fstat(fdin, &statbuf);
 	DIE(rc < 0, "fstat");