rv/rvgen: fix _fill_states() return type annotation

The _fill_states() method returns a list of strings, but the type
annotation incorrectly specified str. Update the annotation to
list[str] to match the actual return value.

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Reviewed-by: Gabriele Monaco <gmonaco@redhat.com>
Link: https://lore.kernel.org/r/20260223162407.147003-20-wander@redhat.com
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>

tools/verification/rvgen/rvgen/ltl2k.py

@@ -75,7 +75,7 @@ class ltl2k(generator.Monitor):
         if not self.name:
             self.name = Path(file_path).stem
 
-    def _fill_states(self) -> str:
+    def _fill_states(self) -> list[str]:
         buf = [
             "enum ltl_buchi_state {",
         ]