[docs] Fix typos for "GitHub" (mantinedev#4692)

Fixed typos for the word "GitHub" in the footer of the docs. The spelling used was "Github" (with a lower-case "h") instead of the correct spelling "GitHub" (with an upper-case "H").

[release] Version: 6.0.19

[release] Version: 6.0.18

[release] Version: 6.0.17

[release] Version: 6.0.16

[release] Version: 6.0.15

[release] Version: 6.0.14

[release] Version: 6.0.13

[release] Version: 6.0.11

[release] Version: 6.0.10