Update readme to indicate using an absolute label for mypy.ini

readme.md

diff --git a/readme.md b/readme.md
index f0f8bfb..978ab96 100644
--- a/readme.md
+++ b/readme.md
@@ -68,7 +68,7 @@
 
 ```starlark
 mypy_aspect = mypy(
-    mypy_ini = "//:mypy.ini",
+    mypy_ini = "@@//:mypy.ini",
     types = types,
 )
 ```