Update discovery-mode.rst

[aunshc]

Updating NGINX integration reference with changes made in v0.115.0 signalfx/splunk-otel-collector#5689.

Requirements

• Content follows Splunk guidelines for style and formatting.
• You are contributing original content.

Describe the change

Automatic discovery for NGINX is now based on the OpenTelemetry receiver - this change reflects the same by linking to the NGINX receiver documentation.

[aurbiztondo-splunk]

Thanks for the updates, @aunshc!