Improve string expansion

Converts the string expansion to an f-string.

mozetl/symbolication/modules_with_missing_symbols.py

@@ -167,9 +167,9 @@ top_missing_with_avail_info = [
     for name, (version, debug_id, debug_file), count in top_missing
 ]
 
+today_date = datetime.today().strftime("%Y-%m-%d")
 subject = (
-    "Weekly report of modules with missing symbols in crash reports: %s"
-    % datetime.today().strftime("%Y-%m-%d")
+    f"Weekly report of modules with missing symbols in crash reports: {today_date}"
 )
 
 body = """