fix(coc): Fix COC installed check

Fixes #70

lua/symbols-outline/providers/coc.lua

@@ -1,12 +1,9 @@
--- local config = require('symbols-outline.config')
-
 local M = {}
 
--- probably change this
 function M.should_use_provider(_)
     local coc_installed = vim.fn.exists("*CocActionAsync")
 
-    if not coc_installed then return end
+    if coc_installed == 0 then return end
 
     local coc_attached = vim.fn.call('CocAction', {'ensureDocument'})
     local has_symbols = vim.fn.call('CocHasProvider', {'documentSymbol'})