docs: use title case for menu items

Author: eddyerburgh

docs/en/SUMMARY.md

@@ -36,12 +36,12 @@
     * [setData](api/wrapper-array/setData.md)
     * [setProps](api/wrapper-array/setProps.md)
     * [trigger](api/wrapper-array/trigger.md)
-  * [selectors](api/selectors.md)
-  * [guides](guides/README.md)
+  * [Selectors](api/selectors.md)
+  * [Guides](guides/README.md)
     * [Introduction to unit tests](guides/introduction-to-unit-tests.md)
     * [Choosing a test runner](guides/choosing-a-test-runner.md)
     * [Using with Jest](guides/using-with-jest.md)
     * [Testing SFCs with Jest](guides/testing-SFCs-with-jest.md)
     * [Testing SFCs with Mocha](guides/testing-SFCs-with-mocha-webpack.md)
     * [Using with Vuex](guides/using-with-vuex.md)  
-  * [common gotchas](common-gotchas.md)
+  * [Common gotchas](common-gotchas.md)