Ignore .virthualenv not .virtualenv

This change was introduced in 3bb4e51184066f5cf60daa029c31e7f52398353b, where it was marked as changed due to a typo. However, that is the correct name of this particular virtual environment. See https://hackage.haskell.org/package/virthualenv

Haskell.gitignore

@@ -6,7 +6,7 @@ cabal-dev
 *.chs.h
 *.dyn_o
 *.dyn_hi
-.virtualenv
+.virthualenv
 .hpc
 .hsenv
 .cabal-sandbox/