Fix typo (#2869)

Author: sepsh

docs/examples/guide/screens/modal01.py

@@ -31,7 +31,7 @@ def on_button_pressed(self, event: Button.Pressed) -> None:
 
 
 class ModalApp(App):
-    """A app with a modal dialog."""
+    """An app with a modal dialog."""
 
     CSS_PATH = "modal01.css"
     BINDINGS = [("q", "request_quit", "Quit")]

docs/examples/guide/screens/modal02.py

@@ -31,7 +31,7 @@ def on_button_pressed(self, event: Button.Pressed) -> None:
 
 
 class ModalApp(App):
-    """A app with a modal dialog."""
+    """An app with a modal dialog."""
 
     CSS_PATH = "modal01.css"
     BINDINGS = [("q", "request_quit", "Quit")]