docs: fix vim.max_lines default in config example

Problem: the vimdoc config example showed max_lines = 500 under vim,
but the actual default in init.lua is 200.

Solution: change the example to match the real default.

doc/diffs.nvim.txt

@@ -66,7 +66,7 @@ Configuration is done via `vim.g.diffs`. Set this before the plugin loads:
         },
         vim = {
           enabled = false,
-          max_lines = 500,
+          max_lines = 200,
         },
         intra = {
           enabled = true,