Remove unnecessary mut

Signed-off-by: Rin Cat (鈴猫) <rincat@rincat.dev>

src/lib.rs

@@ -129,7 +129,7 @@ impl Challenger {
         self.difficulty = (difficulty as f64 * DIFFICULTY_SCALE) as u32;
     }
 
-    pub fn issue_challenge(&mut self) -> String {
+    pub fn issue_challenge(&self) -> String {
         let (_x, y, p) = self.algorithm.create(self.difficulty);
 
         let puzzle = Puzzle {