Antora example tags

Somewhere along the way, several people seem to have got the impression that example code needs tagging with

// #tag[]
// #end[]

instead of simply the line comment idiomatic to the coding language, such as 

`// tag[]`
`# tag[]`
`;; tag[]`
`<! tag[] >`

It's a little jarring for anyone skimming through the code.

Several dozen example files are affected - let's get them fixed for 3.1