Don't auto-correct ebuild commands.

It breaks "ebuild <foo> manifest", as there are Manifest files in the directory.

lib/correction.zsh

@@ -6,3 +6,4 @@ alias mysql='nocorrect mysql'
 alias mkdir='nocorrect mkdir'
 alias gist='nocorrect gist'
 alias heroku='nocorrect heroku'
+alias ebuild='nocorrect ebuild'