doc: add missing ref in flux-config-exec(5)

Problem: The SDEXEC PROPERTIES section is not linked as a cross
reference in flux-config-exec(5), but other sections are.

Use a cross reference for SDEXEC PROPERTIES in flux-config-exec(5).

doc/man5/flux-config-exec.rst

@@ -36,7 +36,7 @@ job-shell
 
 sdexec-properties
    (optional) A table of systemd properties to set for all jobs.  All values
-   must be strings.  See SDEXEC PROPERTIES below.
+   must be strings. See :ref:`sdexec_properties` below.
 
 kill-timeout
    (optional) The amount of time in FSD to wait after ``SIGTERM`` is
@@ -72,6 +72,8 @@ testexec
    **job-exec** test execution implementation (used in mainly for testing).
 
 
+.. _sdexec_properties:
+
 SDEXEC PROPERTIES
 =================