Fix gmt compdef

Fix typo: gm -> gmt

plugins/git/git.plugin.zsh

@@ -82,7 +82,7 @@ alias gwc='git whatchanged -p --abbrev-commit --pretty=medium'
 
 alias gpoat='git push origin --all && git push origin --tags'
 alias gmt='git mergetool --no-prompt'
-compdef _git gm=git-mergetool
+compdef _git gmt=git-mergetool
 
 alias gg='git gui citool'
 alias gga='git gui citool --amend'