Update pubsub.rst

[amaan-ahmad]

Description

Local configuration references specifies the enabled tag, but it is not specified in GCP integration documentation here. Missing this detail was caused a lot of confusion, hence contributing for correction.
Most of the times the dashboard editing option just gave me validation error when I tried to update the configuration without this tag.

Checks

Docs building

• Compiles without warnings.

Code formatting and web optimization

• Uses three spaces indentation.
• Adds or updates meta descriptions accordingly.
• Updates the redirects.js script if necessary (check this guide).

Writing style

• Uses present tense, active voice, and semi-formal registry.
• Uses short, simple sentences.
• Uses bold for user interface elements, italics for key terms or emphasis, and code font for Bash commands, file names, REST paths, and code.