Clarify section title about puma memory use

What does this MR do and why?

In support, we frequently advise customers to raise the default setting for puma per_worker_max_memory_mb. This section in the docs talking about how puma uses and manages memory is where we have to link to give them background and instructions on how to raise the per_worker_max_memory_mb, but the section title is confusing when it is titled "Reducing memory usage". I propose changing this section title to "Tuning memory usage".

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.