Fix Custom Matchers API example message property

CHANGELOG.md

@@ -79,6 +79,7 @@
 
 ### Chore & Maintenance
 
+- `[docs]` Fix message property in custom matcher example to return a function instead of a constant.
 - `[jest-watcher]` Standardize filenames ([#7314](https://github.com/facebook/jest/pull/7314))
 - `[jest-circus]` Standardize file naming in `jest-circus` ([#7301](https://github.com/facebook/jest/pull/7301))
 - `[docs]` Add synchronous test.each setup ([#7150](https://github.com/facebook/jest/pull/7150))

docs/ExpectAPI.md

@@ -107,7 +107,7 @@ expect.extend({
   yourMatcher(x, y, z) {
     return {
       pass: true,
-      message: '',
+      message: () => '',
     };
   },
 });