Fix(docs): update the sendGTMEvent link when referencing gtag.js docs

[p6l-richard]

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What?

Updating an external link pointing to Google Tag Manager docs.

Why?

Because it's written to "refer to google tag manager docs [...] for a full list of variables", but the link pointed to is a dead-end not displaying all parameters.
I therefore updated the link to the gtag.js docs where the parameters are described.

How?

I propose to update the link in the following:
Current link:

Proposed link:

[ijjk]

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[ijjk]

Allow CI Workflow Run

• approve CI run for commit: 5c45e46

Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer