Drop using __file__ to work out the name of the CSS file

See https://github.com/Textualize/textual/pull/963#discussion_r1000546514

Personally I prefer the approach I was using in that it's one less bit of
hard-coded metadata. On the other hand I can appreciate that reducing the
number of possibly-confusing things in an example plays better with people
who may be both new to Textual *and* to Python.

examples/five_by_five.py

@@ -258,7 +258,7 @@ class FiveByFive(App[None]):
     """Main 5x5 application class."""
 
     #: The name of the stylesheet for the app.
-    CSS_PATH = Path(__file__).with_suffix(".css")
+    CSS_PATH = "five_by_five.css"
 
     #: The pre-loaded screens for the application.
     SCREENS = {"help": Help()}