Fix description section Configuration

[HappyFX]

In documentation https://github.com/prometheus/snmp_exporter#prometheus-configuration module variable is a list, but you can't use it like real list.

Working ok:

- job_name: 'working ok'
  params:
      module: [if_mib]

Failed:

- job_name: 'failed 400 error'
  params:
    module: [if_mib,cisco_wlc]

Actual error in prometheus server returned HTTP status 400 Bad Request

Add description about this in configuration:

scrape_configs:
  - job_name: 'snmp'
    static_configs:
      - targets:
        - 192.168.1.2  # SNMP device.
        - switch.local # SNMP device.
    metrics_path: /snmp
    params:
      module: [if_mib]  # can't be used as multi module option one module per job
    relabel_configs:
      - source_labels: [__address__]
        target_label: __param_target
      - source_labels: [__param_target]
        target_label: instance
      - target_label: __address__
        replacement: 127.0.0.1:9116  # The SNMP exporter's real hostname:port.