repo/fs: fixup update task index

src/repo/fs.rs

@@ -95,7 +95,7 @@ impl Repository for FsRepo {
             return Err(Error::NotFound);
         }
 
-        let mut task = &mut tasks[id];
+        let mut task = &mut tasks[id - 1];
         if let Some(name) = update_data.name {
             task.name = name;
         }