[fix] installation instructions - launch SearXNG by python -m (#6078)

[1] https://github.com/searxng/searxng/issues/126#issuecomment-4433874986

Suggested-by: @virtadpt in [1]

docs/build-templates/searxng.rst

@@ -169,7 +169,7 @@ ${fedora_build}
        $ sudo -H -u ${SERVICE_USER} -i
        (${SERVICE_USER})$ cd ${SEARXNG_SRC}
        (${SERVICE_USER})$ export SEARXNG_SETTINGS_PATH=\"${SEARXNG_SETTINGS_PATH}\"
-       (${SERVICE_USER})$ python searx/webapp.py
+       (${SERVICE_USER})$ python -m searx.webapp
 
        # disable debug
        $ sudo -H sed -i -e \"s/debug : True/debug : False/g\" \"$SEARXNG_SETTINGS_PATH\"