Improve wording and "fix example" (remove 3.13) on testing against pre-release (#979)

* Improve wording and "fix example" (remove 3.13) on testing against pre-releases

3.13 was added in a mass tune up in 0b93645e9fea7318ecaed2b359559ac225c90a2b , without
adjusting the wording. With 3.13 listed there too, example does not really make much
sense. So I decided to make it explicit in wording and remove 3.13, so whenever next
refactoring to add 3.14 to be added to every line where 3.13 is -- this would not
even come to attention

* Update to use 3.14 not 3.12 as an example for pre-release