Better wording for "fork me" banner to mention the documentation

[flexjoly]

Hi,

The button 'fork me on github' in the right corner at https://mpdf.github.io/ , goes to the documentation-github and not to the mpdf-github

Greetz, flexJoly

[finwe]

Nope, that refers to the documentation.

[finwe]

Maybe the wording could be improved.