fix and improve token-program course

[wuuer]

Problem

no try/catch for async/await calls, which is not following CONTRIBUTING.md. [Token Extensions Program] link is wrong. [add a second account on devnet] link is wrong. wrong ts command in the section [Create an Associated Token Account to store the tokens]. console.log displays a wrong explorer link due to "!".

Summary of Changes

add try/catch for async/await calls following CONTRIBUTING.md. delete unnecessary "!" for generating a wrong explorer link. add some missing instructions. fix a wrong ts command for the section [Create an Associated Token Account to store the tokens].