Add v before number for consistency

Fixes #852

Author: HonkingGoose

book/02-git-basics/sections/tagging.asc

@@ -276,7 +276,7 @@ do so (now or later) by using -b with the checkout command again. Example:
 
 HEAD is now at 99ada87... Merge pull request #89 from schacon/appendix-final
 
-$ git checkout 2.0-beta-0.1
+$ git checkout v2.0-beta-0.1
 Previous HEAD position was 99ada87... Merge pull request #89 from schacon/appendix-final
 HEAD is now at df3f601... Add atlas.json and cover image
 ----