Fix reading length of undefined at findFirstPage

[10ego]

TypeError: Cannot read property 'length' of undefined
    at exports.findFirstPage (/usr/lib/node_modules/@opendocsg/pdf2md/lib/util/page-number-functions.js:76:38)
    at parse (/usr/lib/node_modules/@opendocsg/pdf2md/lib/util/pdf.js:40:19)
    at async module.exports (/usr/lib/node_modules/@opendocsg/pdf2md/lib/pdf2md.js:22:18)
    at async createMarkdownFiles (/usr/lib/node_modules/@opendocsg/pdf2md/lib/pdf2md-cli.js:55:20)

Loop will continue until firstPage < keys.length, but since keys are zero-based, it leaves the final secondPage at undefined.
This PR attempts to fix it.

[LoneRifle]

lgtm. Going forward, pdf-to-markdown - the package from which pdf2md is derived - may be undergoing major changes and would be more actively maintained than this package. I intend to deprecate this once pdf-to-markdown achieves feature parity.