docs: fix sample code for cmp.config.sources (#942)

doc/cmp.txt

@@ -599,10 +599,11 @@ cmp.config.sources~
     The sources are grouped in the order you specify, and the groups are displayed in a fallback, like chain completion.
 >
     cmp.setup {
-      window = {
-        completion = cmp.config.window.bordered(),
-        documentation = cmp.config.window.bordered(),
-      }
+      sources = cmp.config.sources({
+        { name = 'nvim_lsp' },
+      }, {
+        { name = 'buffer' },
+      })
     }
 <
 cmp.config.window~