README.md - improve the "Multiple statements in one query" section

[paulovieira]

• add links for the official documentation
• escape the backtick character
• change the subtitle to "await sql`.simple()" instead of "await sqlselect 1; select 2;`.simple()" (to be coherent with the other subtitles)
• add a small example below

before

after

[porsager]

Thanks a lot - much better