Update mypy config file examples to use Label

Some mypy label references are used between repos (main and rules_mypy) so it is important that they are declared relative to the original bzl file correctly.  Use `Label()` at the construction site to do this.

(This is the "correct" fix for needing the full canonical name in string form, so remove the advice to use "@@//name")

Author: anguslees

readme.md

@@ -70,14 +70,11 @@ mypy's behavior may be customized using a [mypy config file](https://mypy.readth
 
 ```starlark
 mypy_aspect = mypy(
-    mypy_ini = "@@//:mypy.ini",
+    mypy_ini = Label("//:mypy.ini"),
     types = types,
 )
 ```
 
-> [!NOTE]
-> The label passed to `mypy_ini` needs to be absolute (a prefix of `@@` means the root repo).
-
 > [!NOTE]
 > mypy.ini files should likely contain the following lines to suppress type-checking 3rd party modules.
 >
@@ -107,7 +104,7 @@ And in your `aspects.bzl` (or similar) file:
 load("@rules_mypy//mypy:mypy.bzl", "mypy")
 
 mypy_aspect = mypy(
-    mypy_cli = ":mypy_cli",
+    mypy_cli = Label(":mypy_cli"),
     types = types,
 )
 ```