Correct a management UI metric description

[michaelklishin]

Proposed Changes

This corrects a management UI help hint.

Types of Changes

• Bug fix (non-breaking change which fixes issue Incorrect help text displayed when clicking on the "?" button of the "Memory" column (in the "Overview" tab, "Nodes" table) #4493)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
• I have added necessary documentation to https://github.com/rabbitmq/rabbitmq-website

Further Comments

Closes #4493