readme: fix the layout:include example

When I copy-pasted this example as-is into an app to try it out, it didn’t work.
Instead of showing “Message goes here!”, it was showing “My modal content”.
Based on the comments in ticket #5 and my own testing, I believe that this
should have been layout:fragment rather than th:fragment.
Merge pull request #35 from davidmc24/feature/fix-readme-include-example

readme: fix the layout:include example
Merge branch 'dev'
Create 1.2.3 release
Remove th namespace from title examples (not needed, make simpler)
Prepare for 1.2.4 development.