fix(google-tag-manager): rename pluginGoogleAnalytics to pluginGoogleTagManager

[akshatsinha0]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

The default export function in docusaurus-plugin-google-tag-manager/src/index.ts is named
pluginGoogleAnalytics. This is a copy-paste artifact from docusaurus-plugin-google-analytics, where
that name is correct.

The other two Google plugins follow the correct naming convention:
- docusaurus-plugin-google-analytics -> pluginGoogleAnalytics
- docusaurus-plugin-google-gtag -> pluginGoogleGtag
- docusaurus-plugin-google-tag-manager -> pluginGoogleAnalytics {wrong}

The plugin's name property already returns the correct string
'docusaurus-plugin-google-tag-manager'. Only the function identifier we looked into.

Test Plan

- No behavioral change. The function is a default export, so no external code references it by name.  

Verified that no other file in therepo imports this function by identifier.

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

N/A

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