Fix rbs validate warning

```
❯ bundle exec rbs -I sig validate
W, [2024-06-07T16:09:23.137310 #4825]  WARN -- rbs: sig/lrama/grammar.rbs:74:19...74:37: `void` type is only allowed in return type or generics parameter (RBS::WillSyntaxError)

      def validate!: () -> (void | bot)
                     ^^^^^^^^^^^^^^^^^^
```

sig/lrama/grammar.rbs

@@ -71,7 +71,7 @@ module Lrama
     def epilogue_first_lineno=: (Integer epilogue_first_lineno) -> Integer
     def epilogue=: (String epilogue) -> String
     def prepare: () -> void
-    def validate!: () -> (void | bot)
+    def validate!: () -> void
     def find_rules_by_symbol!: (Grammar::Symbol sym) -> Array[Rule]
     def find_rules_by_symbol: (Grammar::Symbol sym) -> Array[Rule]?