gui/manual/intro.txt interpunction
This patch is split from D2305. It does the following:
- replace programmer's quotes () in text with proper opening and closing quotation marks ()
- start sentences with a capital
- →
- remove an incorrect colon ()
- use imperative
Author: Nescio
Reviewed By: Gallaecio, Freagarach
Differential Revision: https://code.wildfiregames.com/D2560