manual: 'Running': flatten section depths a bit

I'd like to add more sub-levels in an upcoming commit, and the levels
are becoming too many. So let's get rid of one depth level in the
'Running osmo-upf' chapter.

Change-Id: I0bd43300aa4b45315ea58ab35c77da005d1a4fa4

doc/manuals/chapters/running.adoc

@@ -59,9 +59,7 @@ tunmap
  table-name osmo-upf-2
 ----
 
-=== Configuring Primary Links
-
-==== Configure PFCP Server
+=== Configure PFCP Server
 
 The following example configures OsmoUPF to listen for PFCP association requests
 from Control Plane Function entities on local interface 10.9.8.7, port 8805:
@@ -83,7 +81,7 @@ ID, and so far uses the 'local-addr' as local Node ID -- hence the 'local-addr'
 must not be "0.0.0.0", which is an unfortunate consequence. This is likely to
 improve in the future, see https://osmocom.org/issues/5682 .
 
-==== Configure Linux Kernel GTP Features
+=== Linux Kernel Features
 
 OsmoUPF uses two distinct Linux kernel features:
 
@@ -103,7 +101,7 @@ GTP kernel module configuration can be omitted for sites that serve only as GTP
 forwarding proxy, without encapsulation/decapsulation of GTP payloads.
 
 [[gtp_module]]
-===== Configure Linux Kernel GTP Module for `tunend`
+=== Configure Linux Kernel GTP Module for `tunend`
 
 The Linux kernel GTP module is used for the `tunend` use case, i.e. GTP
 encapsulation/decapsulation from/to "the internet".
@@ -160,7 +158,7 @@ the `dev` config. In this case, all Network Instance names will be served by
 this GTP device. When using ANY, there should be exactly one GTP dev configured.
 
 [[nftables]]
-===== Configure Linux netfilter for `tunmap`
+=== Configure Linux netfilter for `tunmap`
 
 The Linux kernel netfilter module is used for GTP tunnel proxying, also known as
 tunnel forwarding or tunnel mapping.