Update comment

src/main.rs

@@ -21,7 +21,7 @@ fn kernel_main(boot_info: &'static BootInfo) -> ! {
     let mut mapper = unsafe { memory::init(phys_mem_offset) };
     let mut frame_allocator = unsafe { BootInfoFrameAllocator::init(&boot_info.memory_map) };
 
-    // map a previously unmapped page
+    // map an unused page
     let page = Page::containing_address(VirtAddr::new(0xdeadbeaf000));
     memory::create_example_mapping(page, &mut mapper, &mut frame_allocator);