fixes

01-problem/src/main.rs

@@ -13,7 +13,7 @@ impl CredentialManager {
         }
     }
 
-    pub fn unlock(&self, master_secret: &str) {
+    pub fn unlock(&mut self, master_secret: &str) {
         todo!()
     }
 
@@ -21,11 +21,11 @@ impl CredentialManager {
         todo!()
     }
 
-    pub fn get(&mut self, key: &str) -> Option<&str> {
+    pub fn get(&self, key: &str) -> Option<&str> {
         todo!()
     }
 
-    pub fn lock(&self) {
+    pub fn lock(&mut self) {
         todo!()
     }
 }