doc: Fix reference to salt executors
doc: Fix reference to salt executors

sphinx shows a warning:

doc/topics/development/modules/index.rst:133: WARNING: undefined label:
all-salt_executors (if the link has no caption the label must precede a section
header)

The executors reference is called all-salt.executors (with a dot instead of an
underscore).

Signed-off-by: Benjamin Drung <86a6db991ec5c1a96f98c92b98df9d547d7c8c84@cloud.ionos.com>

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt