Use a byte char instead of casting

src/vga_buffer.rs

@@ -91,7 +91,7 @@ impl Writer {
 
     fn clear_row(&mut self, row: usize) {
         let blank = ScreenChar {
-            ascii_character: ' ' as u8,
+            ascii_character: b' ',
             color_code: self.color_code,
         };
         self.buffer().chars[row] = [blank; BUFFER_WIDTH];