webapp: fix the inbound email command name

the webapp declare the command using _, puppet with -

Related to T4258

Test Plan

• webapp staging

diff origin/production/webapp.internal.staging.swh.network current/webapp.internal.staging.swh.network
*******************************************
  Mailalias[swhwebapp in /etc/postfix/swhwebapp/aliases] =>
   parameters =>
     recipient =>
      - ["| django-admin process-inbound-email --settings=swh.web.settings.production"]
      + ["| django-admin process_inbound_email --settings=swh.web.settings.production"]
*******************************************
*** End octocatalog-diff on webapp.internal.staging.swh.network

• moma

diff origin/production/moma.softwareheritage.org current/moma.softwareheritage.org
*******************************************
  Mailalias[swhwebapp in /etc/postfix/swhwebapp/aliases] =>
   parameters =>
     recipient =>
      - ["| django-admin process-inbound-email --settings=swh.web.settings.production"]
      + ["| django-admin process_inbound_email --settings=swh.web.settings.production"]
*******************************************
*** End octocatalog-diff on moma.softwareheritage.org

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Migrated from D7849 (view on Phabricator)