fix: replaced unnecessary &mut self with &self

src/cpu.rs

@@ -97,7 +97,7 @@ impl Cpu {
         self.bus.read_word(addr)
     }
 
-    pub fn read_byte(&mut self, addr: u16) -> u8 {
+    pub fn read_byte(&self, addr: u16) -> u8 {
         self.bus.read_byte(addr)
     }