Update screenshot for Collect navigation buttons

[lognaturel]

https://docs.opendatakit.org/collect-filling-forms/#left-and-right-buttons

The buttons how now been updated to have a Material design look and feel.

[gaurav-goswamii]

It looks like this issue was addressed by #1103, which was ultimately resolved in #1285. Since #1285 has been merged and closed, I believe this issue can also be closed.