fix(colubris-snmp): fix labels to manage 15 minutes average thresholds

[omercier]

Description

Fixed the counter label for 15 minutes average load.
The wrong label caused the plugin to refuse --warning-15min options.

Refs: CTOR-1197

Type of change

• Patch fixing an issue (non-breaking change)
• New functionality (non-breaking change)
• Functionality enhancement or optimization (non-breaking change)
• Breaking change (patch or feature) that might cause side effects breaking part of the Software

Checklist

• I have followed the coding style guidelines provided by Centreon
• I have commented my code, especially hard-to-understand areas of the PR.
• I have rebased my development branch on the base branch (develop).
• In case of a new plugin, I have created the new packaging directory accordingly.
• I have implemented automated tests related to my commits.
  • Data used for automated tests are anonymized.
• I have reviewed all the help messages in all the .pm files I have modified.
  • All sentences begin with a capital letter.
  • All sentences end with a period.
  • I am able to understand all the help messages, if not, exchange with the PO or TW to rewrite them.
• After having created the PR, I will make sure that all the tests provided in this PR have run and passed.