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.

README.rst

@@ -136,7 +136,7 @@ After the RFC is ready to go, please announce it to `arch-dev-public
 
 Here is a template:
 
-.. code::
+.. code:: email
 
   Subject: RFC: Deprecate BadFeature
 
@@ -201,7 +201,7 @@ affirmation). The Final Comment Period should also be announced on
 
 Here is a template:
 
-.. code::
+.. code:: email
 
   Subject: RFC Final Comment Period: Deprecate BadFeature