README.rst: Define code blocks to have `email` language
This avoids a CRITICAL error from rstcheck.
CRITICAL:rstcheck_core.checker:An `AttributeError` error occured.
This is most propably due to a code block directive (code/code-
block/sourcecode) without a specified language. This may result in a
false negative for source: 'README.rst'. See
https://rstcheck-core.readthedocs.io/en/latest/faq/#code-blocks-without-language-sphinx
for more information.