Fix custom matcher example (#56)
Cannot be a const constructor
diff --git a/lib/src/core_matchers.dart b/lib/src/core_matchers.dart
index 94f9d7d..a3e4f1b 100644
--- a/lib/src/core_matchers.dart
+++ b/lib/src/core_matchers.dart
@@ -595,15 +595,18 @@
/// have a Widget class where each Widget has a price; we could make a
/// [CustomMatcher] that can make assertions about prices with:
///
-/// class HasPrice extends CustomMatcher {
-/// const HasPrice(matcher) :
-/// super("Widget with price that is", "price", matcher);
-/// featureValueOf(actual) => actual.price;
-/// }
+/// ```dart
+/// class HasPrice extends CustomMatcher {
+/// HasPrice(matcher) : super("Widget with price that is", "price", matcher);
+/// featureValueOf(actual) => actual.price;
+/// }
+/// ```
///
/// and then use this for example like:
///
-/// expect(inventoryItem, new HasPrice(greaterThan(0)));
+/// ```dart
+/// expect(inventoryItem, new HasPrice(greaterThan(0)));
+/// ```
class CustomMatcher extends Matcher {
final String _featureDescription;
final String _featureName;
