[3.13] gh-120276: Fix incorrect email.header.Header maxlinelen default (GH-120277) (GH-120279)

(cherry picked from commit 7c016de)

Co-authored-by: Clinton <[email protected]>

Author: miss-islington

Doc/library/email.header.rst

@@ -77,7 +77,7 @@ Here is the :class:`Header` class description:
    The maximum line length can be specified explicitly via *maxlinelen*.  For
    splitting the first line to a shorter value (to account for the field header
    which isn't included in *s*, e.g. :mailheader:`Subject`) pass in the name of the
-   field in *header_name*.  The default *maxlinelen* is 76, and the default value
+   field in *header_name*.  The default *maxlinelen* is 78, and the default value
    for *header_name* is ``None``, meaning it is not taken into account for the
    first line of a long, split header.