Fix card on main page for consistency

Change "To API docs" to "To the API docs", since
that is more consistent with the other two cards, i.e.
"To the user guide" and "To the developer guide".

docs/index.rst

@@ -117,4 +117,4 @@ Prototype CTA Pipeline Framework (``ctapipe``)
             :color: primary
             :click-parent:
 
-            To API docs
+            To the API docs