DOC: Convert code-block to ipython block in io.rst

[mroeschke]

Pandas version checks

• I have checked that the issue still exists on the latest versions of the docs on master here

Location of the documentation

doc/source/user_guide/io.rst

Documentation problem

xref #45146 (comment)

.. :code-block: should be converted to .. :ipython:

Suggested fix for documentation

.. :code-block: should be converted to .. :ipython: