Don't include the shell prompt in "Copy" links for guides

[cowboyd]

When following the installation process in our guides in #882 @zburke noted that the "copy" links to copy commands included the shell prompt, which is not super useful since you end up with a double shell prompt when you try to use it.

In other words, in the display $ npm install bigtest --save-dev, the string that should be copied is just the command itself:

npm install bigtest --save-dev

not the command plus shell prompt.

$ npm install bigtest --save-dev

[jnicklas]

Can we just get rid of the leading $? Including it in the shell prompt is just a convention anyway. It use to be to distinguish from running as root using su where it would show a leading #, but no one ever does this these days anyway. My prompt does not include the $ at all for example: