docs: add missing definition for space
docs: add missing definition for space

The definition for space was missing in PLUGIN_AUTHORING documentation.
Added ' ' for defining a space, as \s.

Signed-off-by: Saija Sorsa <5c3aef4b51b169a958f33f14950302c329e3f706@intel.com>

intelsdi-x/snap,mjbrender/snap,sandlbn/snap,intelsdi-x/snap,mjbrender/snap,sandlbn/snap