git: fix version check

plugins/git/git.plugin.zsh

@@ -1,6 +1,6 @@
 # Git version checking
 autoload -Uz is-at-least
-git_version="${(As: :)$(git version 2>/dev/null)[3]}"
+git_version="${${(As: :)$(git version 2>/dev/null)}[3]}"
 
 #
 # Functions