[SNMP] Fix config template issue when setting snmpagentaddress (sonic-net#21987)

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Related to [PR#16187](sonic-net#16187)

#### Why I did it

When you configure an SNMP agent address with a VRF, the snmpd service will fail to properly start after the configuration has been applied. The snmpd service shows a status of "FATAL."

```
sudo config snmpagentaddress add 1.1.1.1 -p 161 -v mgmt

docker exec -it snmp supervisorctl status | awk '{print $1, $2}'
dependent-startup RUNNING
rsyslogd RUNNING
snmp-subagent STOPPED
snmpd FATAL
start EXITED
supervisor-proc-exit-listener RUNNING
```

##### Work item tracking
- Microsoft ADO **(number only)**:

#### How I did it
Corrected syntax in the jinja template used to generate the snmpd.conf within the SNMP docker container.

#### How to verify it
```
sudo config snmpagentaddress add 1.1.1.1 -p 161 -v mgmt

docker exec -it snmp supervisorctl status | awk '{print $1, $2}'
dependent-startup EXITED
rsyslogd RUNNING
snmp-subagent RUNNING
snmpd RUNNING
start EXITED
supervisor-proc-exit-listener RUNNING
```

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Author: mssonicbld

dockers/docker-snmp/snmpd.conf.j2

@@ -26,7 +26,7 @@
 
 {% if SNMP_AGENT_ADDRESS_CONFIG %}
 {% for (agentip, port, vrf) in SNMP_AGENT_ADDRESS_CONFIG %}
-agentAddress {{ protocol(agentip) }}:[{{ agentip }}]{% if port %}:{{ port }}{% endif %}{% if vrf %}%{{ vrf }}{% endif %}{{ "" }}
+agentAddress {{ protocol(agentip) }}:[{{ agentip }}]{% if vrf %}@{{ vrf }}{% endif %}{% if port %}:{{ port }}{% endif %}{{ "" }}
 {% endfor %}
 {% else %}
 agentAddress udp:161