Use uppercase example class names in meta

Change-Id: I21f4ff480648cd0e2e3f732fd56ec6e3564d93fc
Reviewed-on: https://dart-review.googlesource.com/71230
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>
diff --git a/pkg/meta/lib/meta.dart b/pkg/meta/lib/meta.dart
index d59cc28..38a8283 100644
--- a/pkg/meta/lib/meta.dart
+++ b/pkg/meta/lib/meta.dart
@@ -193,13 +193,13 @@
 /// Classes in the same package as the marked class or mixin may extend,
 /// implement or mix-in the annotated class or mixin.
 ///
-/// Given a class or mixin `c`, which is annotated with this, and a class or
-/// mixin `d`, which extends, implements, or mixes in `c`, then tools, such as
+/// Given a class or mixin `C`, which is annotated with this, and a class or
+/// mixin `D`, which extends, implements, or mixes in `C`, then tools, such as
 /// the analyzer, may emit warnings if:
 ///
-/// * `c` and `d` are declared in different packages, or
-/// * `c` and `d` are declared in the same package, and `d` is not also
-/// annotated with this.
+/// * `C` and `D` are declared in different packages, or
+/// * `C` and `D` are declared in the same package, and `D` is not also
+///   annotated with this.
 const _Sealed sealed = const _Sealed();
 
 /// Used to annotate a field that is allowed to be overridden in Strong Mode.