Fix id of 'Internet Message Format' RFC in email doc (GH-24137)

Previous ID (5233) refers to "Sieve Email Filtering: Subaddress
Extension". It seems that the actual reference should be "Internet
Message Format" RFC 5322 (https://tools.ietf.org/html/rfc5322).

(The typo probably comes from commit 29d1bc0842 in which the ID of
this RFC got updated from the obsolete 2822.)

Co-authored-by: Ambrose Chua <ambrose@hey.com>