sh.1: Fix rendering error due to redundant .El

Due to this all the rest of the items in the Built-in Commands section
were not rendered at all.

Fixes: 2711852bd9ac ("sh.1: Provide detailed job control documentation")
MFC after: 3 days
Reviewed by: emaste, ziaee
Differential Revision: https://reviews.freebsd.org/D55080