docs: explicitly specify plaintext format

src/certs/file_sscv.rs

@@ -13,7 +13,7 @@ pub struct KnownHostsFile {
 impl KnownHostsFile {
     /// Read or create a known_hosts file at the given path.
     /// Format of known_hosts is: _(fields are separated by any space sequence)_
-    /// ```
+    /// ```text
     /// # host  expires    algo   base64 fingerprint
     /// dc09.ru 1722930541 sha512 dGVzdHRlc3R0ZXN0Cg
     /// ```