docs(boxing_todo): fix typo

subjects/boxing_todo/README.md

@@ -157,7 +157,7 @@ And its output:
 ```console
 $ cargo run
 TodoList { title: "TODO LIST FOR PISCINE RUST", tasks: [Task { id: 0, description: "do this", level: 0 }, Task { id: 1, description: "do that", level: 5 }] }
-Fail to parses todoNone
+Fail to parse todoNone
 Fail to parse todo Some(Malformed(UnexpectedCharacter { ch: ',', line: 2, column: 18 }))
 $
 ```