docs(digits): fix update docstring raises type

Fix the docstring for `Digits.update`, which raises a TypeError rather
than a ValueError if the value is not a str.

src/textual/widgets/_digits.py

@@ -6,6 +6,7 @@
 
 if TYPE_CHECKING:
     from textual.app import RenderResult
+
 from textual.geometry import Size
 from textual.renderables.digits import Digits as DigitsRenderable
 from textual.widget import Widget
@@ -59,7 +60,7 @@ def update(self, value: str) -> None:
             value: New value to display.
 
         Raises:
-            ValueError: If the value isn't a `str`.
+            TypeError: If the value isn't a `str`.
         """
         if not isinstance(value, str):
             raise TypeError("value must be a str")