[RFC] Fix last non-portable format specifier

Before, I said there remained a single instance of unportable format string at https://github.com/neovim/neovim/blob/8f710a4103607c3568b31bc9b1002aafc6954489/src/mark.c#L1497
I had left that to be done when type refactoring was dealt with, because it was a sscanf.

Now, on [suggestion](https://github.com/neovim/neovim/issues/490#issuecomment-41316437) from @philix, I saw I could easily fix it too.

So, with this, there are no more non-portable format specifiers in the code.