Fix kak_sessions not sending _values enough arguments

Using zsh 5.4.2, I am occasionally running into an error "_values:compvalues:11: not enough arguments". When it doesn't occur, instead, one of the kakoune sessions does not appear as part of autocompletion.

This fixes both problems for me by providing a description as described in http://zsh.sourceforge.net/Doc/Release/Completion-System.html#Completion-System. "The first non-option argument is used as a string to print as a description before listing the values. "

src/_kak

@@ -43,7 +43,7 @@
 
 _kak_sessions() {
   session_ids=($(_call_program session_ids kak -l))
-  _values "${session_ids[@]}"
+  _values 'kak sessions' "${session_ids[@]}"
 }
 
 _kak() {