style(p4, p9): fix style issue

problems/p4/src/main.rs

@@ -13,10 +13,10 @@ fn main() -> Result<(), Box<dyn Error>> {
         // 对密文的每个字节应用异或操作，得到明文
         let plaintext: Vec<u8> = cipher_bytes.iter().map(|&byte| byte ^ key).collect();
         for i in 0..=(plaintext.iter().len() - window_size) {
-            if let Ok(text) = std::str::from_utf8(&plaintext[i..i + window_size]) {
-                if is_valid_english(text, None) {
-                    println!("Found valid sentence with key {key}: {text}");
-                }
+            if let Ok(text) = std::str::from_utf8(&plaintext[i..i + window_size])
+                && is_valid_english(text, None)
+            {
+                println!("Found valid sentence with key {key}: {text}");
             }
         }
         // 尝试将字节转换为字符串