Fix incorrect link syntax in upgrade guide (#5434)

Looks like some markdown spilled into our restructured text

docs/upgrade.rst

@@ -24,7 +24,8 @@ changes are that:
   function is not available anymore.
   function is not available anymore.
 
 
 Due to NumPy changes, you may experience difficulties updating to NumPy 2.
 Due to NumPy changes, you may experience difficulties updating to NumPy 2.
-Please see the [NumPy 2 migration guide](https://numpy.org/devdocs/numpy_2_0_migration_guide.html) for details.
+Please see the `NumPy 2 migration guide <https://numpy.org/devdocs/numpy_2_0_migration_guide.html>`_
+for details.
 For example, a more direct change could be that the default integer ``"int_"``
 For example, a more direct change could be that the default integer ``"int_"``
 (and ``"uint"``) is now ``ssize_t`` and not ``long`` (affects 64bit windows).
 (and ``"uint"``) is now ``ssize_t`` and not ``long`` (affects 64bit windows).