content: Use weightVariableTextStyle in ContentTheme.textStyleError

This converts the last remaining place where we were using
TextStyle.fontWeight directly, when we should have been using
weightVariableTextStyle.

(In a future with
  flutter/flutter#148026
, though, all uses of weightVariableTextStyle should be changed back
to plain TextStyle.fontWeight.)

lib/widgets/content.dart

@@ -47,8 +47,8 @@ class ContentTheme extends ThemeExtension<ContentTheme> {
     )
       .merge(weightVariableTextStyle(context))
       .copyWith(debugLabel: 'ContentTheme.textStylePlainParagraph'),
-    textStyleError = const TextStyle(
-      fontSize: kBaseFontSize, fontWeight: FontWeight.bold, color: Colors.red);
+    textStyleError = const TextStyle(fontSize: kBaseFontSize, color: Colors.red)
+      .merge(weightVariableTextStyle(context, wght: 700));
 
   ContentTheme._({
     required this.textStylePlainParagraph,