revert from :py:code: to `` (#1643)

Author: bendichter

docs/source/export.rst

@@ -31,7 +31,7 @@ on the :py:class:`~pynwb.file.NWBFile` before exporting.
     Modifications to :py:class:`h5py.Dataset <h5py.Dataset>` objects act *directly* on the read file on disk.
     Changes are applied immediately and do not require exporting or writing the file. If you want to modify a dataset
     only in the new file, than you should replace the whole object with a new array holding the modified data. To
-    prevent unintentional changes to the source file, the source file should be opened with :py:code:`mode='r'`.
+    prevent unintentional changes to the source file, the source file should be opened with ``mode='r'``.
 
 .. note::