Correct guide/index.md typo
Correct guide/index.md typo

are instaniated -> aren't instantiated
kfreiman/riot.github.io,riot/riot.github.io,kfreiman/riot.github.io,narikei/riot.github.io,kfreiman/riot.github.io,sylvainpolletvillard/riot.github.io,riot/riot.github.io,sylvainpolletvillard/riot.github.io,narikei/riot.github.io,riot/riot.github.io,sylvainpolletvillard/riot.github.io,narikei/riot.github.io