feh.1: Silence mandoc warnings

This fixes the following mandoc warnings.

  man -Tlint feh

  WARNING: skipping empty macro: No

  skipping empty macro
    (mdoc) The indicated macro has no arguments and hence no effect.

  man: /tmp/man1/feh.1:16:14: WARNING: skipping empty macro: No
  man: /tmp/man1/feh.1:16:34: WARNING: skipping empty macro: No
  man: /tmp/man1/feh.1:179:2: WARNING: skipping empty macro: No
  man: /tmp/man1/feh.1:387:48: WARNING: skipping empty macro: No
  man: /tmp/man1/feh.1:785:26: WARNING: skipping empty macro: No
  man: /tmp/man1/feh.1:785:38: WARNING: skipping empty macro: No
  man: /tmp/man1/feh.1:823:26: WARNING: skipping empty macro: No