Update README.md

vain -> vein

https://english.stackexchange.com/questions/167641/in-the-same-vein-as-vs-in-a-similar-vein-to-vs-along-a-similar-vein

Author: sralmai

README.md

@@ -168,7 +168,7 @@ Alternatives:
 maybe
 -----
 
-In a similar vain to `true` and `false` the `maybe` command exits with
+In a similar vein to `true` and `false` the `maybe` command exits with
 a status code of zero or one, depending on a random number.
 
 It can be useful in scripts which need to test-failures, or which benefit