chore: correct StyleCI fixer name to nullable_type_declarations

Author: imorland

.styleci.yml

@@ -5,7 +5,7 @@ enabled:
 
 disabled:
   - align_double_arrow
-  - nullable_type_declaration
+  - nullable_type_declarations
   - blank_line_after_opening_tag
   - multiline_array_trailing_comma
   - new_with_braces