Fix error that was hidden in zpty

Without the quotes, this was blowing up with:
  _zsh_autosuggest_capture_postcompletion:unset:5: not enough arguments

src/strategies/completion.zsh

@@ -10,7 +10,7 @@ _zsh_autosuggest_capture_postcompletion() {
 	compstate[insert]=1
 
 	# Don't list completions
-	unset compstate[list]
+	unset 'compstate[list]'
 }
 
 _zsh_autosuggest_capture_completion_widget() {

zsh-autosuggestions.zsh

@@ -492,7 +492,7 @@ _zsh_autosuggest_capture_postcompletion() {
 	compstate[insert]=1
 
 	# Don't list completions
-	unset compstate[list]
+	unset 'compstate[list]'
 }
 
 _zsh_autosuggest_capture_completion_widget() {