Update footer's GitHub repository URL and capitalisation of "GitHub" (#1983)
Update footer's GitHub repository URL and capitalisation of "GitHub" (#1983)

This updates the Prism repository linked to in the footer and corrects the capitalization of the name "GitHub".
PrismJS/prism,PrismJS/prism,PrismJS/prism,PrismJS/prism,PrismJS/prism