tidy: improve comment

Author: hkBst

src/tools/tidy/src/fluent_used.rs

@@ -12,7 +12,7 @@ fn filter_used_messages(
 ) {
     // we don't just check messages never appear in Rust files,
     // because messages can be used as parts of other fluent messages in Fluent files,
-    // so we do checking messages appear only once in all Rust and Fluent files.
+    // so we check messages appear only once in all Rust and Fluent files.
     let matches = static_regex!(r"\w+").find_iter(contents);
     for name in matches {
         if let Some((name, filename)) = msgs_not_appeared_yet.remove_entry(name.as_str()) {