doc: the build opt must return a string value

The `build` opt must be fun(LazyPlugin) or string or false or a list of
build commands, not fun():string

Ref: https://lazy.folke.io/spec

README.md

@@ -75,14 +75,9 @@ For building binary if you wish to build from source, then `cargo` is required.
   "yetone/avante.nvim",
   -- if you want to build from source then do `make BUILD_FROM_SOURCE=true`
   -- ⚠️ must add this setting! ! !
-  build = function()
-    -- conditionally use the correct build system for the current OS
-    if vim.fn.has("win32") == 1 then
-      return "powershell -ExecutionPolicy Bypass -File Build.ps1 -BuildFromSource false"
-    else
-      return "make"
-    end
-  end,
+  build = vim.fn.has("win32")
+      and "powershell -ExecutionPolicy Bypass -File Build.ps1 -BuildFromSource false"
+      or "make",
   event = "VeryLazy",
   version = false, -- Never set this value to "*"! Never!
   ---@module 'avante'