CMSG_DATA.3: Fix formatting of printf escape sequences

Escape sequences like "\n" have to be escaped twice in examples in our
mdoc(7)-based manual pages in order to be displayed properly. The problem
is that otherwise they are interpreted by mdoc(7), which results in:

printf("parent: received '%s'0, buf);

being shown to the user instead of:

printf("parent: received '%s'\n", buf);

Approved by: bcr (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24056