[clang-format] Open plurals.txt in read-only mode in dump_format_style.py

Author: owenca

clang/docs/tools/dump_format_style.py

@@ -20,7 +20,7 @@
 PLURALS_FILE = os.path.join(os.path.dirname(__file__), "plurals.txt")
 
 plurals: Set[str] = set()
-with open(PLURALS_FILE, "a+") as f:
+with open(PLURALS_FILE) as f:
     f.seek(0)
     plurals = set(f.read().splitlines())