Add note about trailing slash in the base option

[neuberoliveira]

I have trouble with images not being loaded in PDF, reading about in #44 i realize that base option must have a trailing slash, as i stated in this comment https://github.com/marcbachmann/node-html-pdf/issues/44#issuecomment-1051090566

Just add a note to let it crystal clear, its easy to miss the /at the end