TT#99002 SNMP SIP metrics: fix syntax for sipConcurrentCalls

Fixes:

| root@sp1:~# goss -g /etc/ngcp-system-tests/915_service-snmp.yaml validate
| Error: could not unmarshal "command:\n\n  snmpwalk -v2c -cpublic localhost sipwise:\n    [...] as YAML data: yaml: line 63: found unknown escape character

Change-Id: Ibd25dc9fbe6d67632893ceb02dde54f9b6b522f4
Fixes: commit d6f5861edf

templates/915_service-snmp.yaml.tt2

@@ -75,7 +75,7 @@ command:
     - "/::sipNetworkEfficiencyRatio\\.\\d = /"
 [% END -%]
 [% IF is_proxy -%]
-    - "/::sipConcurrentCalls\.\\d = /"
+    - "/::sipConcurrentCalls\\.\\d = /"
     - "/::sipDialogEarlyMedia\\.\\d = /"
     - "/::sipDialogActive\\.\\d = /"
     - "/::sipDialogLocal\\.\\d = /"