Updating rust doc

```
Restore from the latest backup

Arguments
```
db_dir - A path to the database directory
wal_dir - A path to the wal directory
opts - Restore options
```
Example
```
ⓘ
use rocksdb::backup::{BackupEngine, BackupEngineOptions};
let backup_opts = BackupEngineOptions::default();
let backup_engine = BackupEngine::open(&backup_opts, &backup_path).unwrap();
let mut restore_option = rocksdb::backup::RestoreOptions::default();
restore_option.set_keep_log_files(true); /// true to keep log files
if let Err(e) = backup_engine.restore_from_latest_backup(&db_path, &wal_dir, &restore_option) {
    error!("Failed to restore from the backup. Error:{:?}", e);
    return Err(e.to_string());
 }
```

src/backup.rs

@@ -79,10 +79,16 @@ impl BackupEngine {
         }
     }
     /// Restore from the latest backup
-    /// db_dir:  input db directory
+    ///
-    /// wal_dir:  input wal directory
+    /// # Arguments
-    /// opts: restore options
+    ///
-    /// example:
+    /// * `db_dir` - A path to the database directory
+    /// * `wal_dir` - A path to the wal directory
+    /// * `opts` - Restore options
+    ///
+    /// # Example
+    ///
+    /// ```ignore
     /// use rocksdb::backup::{BackupEngine, BackupEngineOptions};
     /// let backup_opts = BackupEngineOptions::default();
     /// let backup_engine = BackupEngine::open(&backup_opts, &backup_path).unwrap();
@@ -92,6 +98,7 @@ impl BackupEngine {
     ///     error!("Failed to restore from the backup. Error:{:?}", e);
     ///     return Err(e.to_string());
     ///  }
+    /// ```
     pub fn restore_from_latest_backup<D: AsRef<Path>, W: AsRef<Path>>(
         &mut self,
         db_dir: D,