chore[test]: fix a type hint (#4173)

Description
Fix a type check warning reported by Pyre@Google, which was outdated
after code modifications.

Detail
update the return type of function fix_terminal from bool to str, since
it could be str after commit 176e7f7

tests/functional/grammar/test_grammar.py

@@ -37,7 +37,7 @@ def test_basic_grammar_empty():
     assert len(tree.children) == 0
 
 
-def fix_terminal(terminal: str) -> bool:
+def fix_terminal(terminal: str) -> str:
     # these throw exceptions in the grammar
     for bad in ("\x00", "\\ ", "\x0c"):
         terminal = terminal.replace(bad, " ")