[Typo] Wrong ndk version guide in 'Prerequisites' page

[JIHOI-KIM]

Typo in Prerequisites
----------------- Now -------------------
NDK r91c+ is required to build Android games (tested with r19c) May be called 19.2.xx from within Android Studio

I don't think 'r91c' version exist. I hope it is typo of 'r19c'.

[slackmoehrle]

Thank you.