zsh:compl: Remove unused git-signoff completion

The shell function git-signoff was replaced with gits builtin
functionality in 09ef4d4d93 ("git: Replace git-signoff with `rebase
--signoff`").

Also adjust the example in the new commment.

.config/zsh/zshrc.d/20-completion.zsh

@@ -45,16 +45,15 @@ compdef nvim-man=man
 #       <C-x>?
 
 # Define completion functions instead of directly linking (e.g. `compdef
-# _git-rebase git-signoff`) so that the completion works for the shell functions
-# **as well** as git aliases (i.e. `git-signoff` and `git signoff`).
+# _git-checkout git-checkout-worktree`) so that the completion works for the
+# shell functions **as well** as for git aliases (i.e. `git-checkout-worktree`
+# and `git cow`).
 _git-checkout-worktree() { _git-checkout }
 _git-commit-last-msg() { _git-commit }
-_git-signoff() { _git-rebase }
 
 compdef _git-log glog
 compdef _git-checkout-worktree git-checkout-worktree
 compdef _git-commit-last-msg git-commit-last-msg
-compdef _git-signoff git-signoff
 
 # Run git's completion once to avoid a `command not found` error when using the
 # completion functions on other programs without having completed something for