fix: correct branch in thirdweb template links (main -> master)

[haileymarshall]

What changed? Why?

The Thirdweb integration guide links to the base-account-thirdweb-template in the base/demos repo using a /tree/main/ URL, but that repo's default branch is master, so both links 404:

• https://github.com/base/demos/tree/main/base-account/base-account-thirdweb-template → 404
• https://github.com/base/demos/tree/master/base-account/base-account-thirdweb-template → 200

This updates both occurrences in docs/base-account/framework-integrations/thirdweb.mdx (the inline link and the GithubRepoCard) from main to master.

Notes to reviewers

• The other 9 base/demos/tree/... links across the docs already correctly use master; these two were the only ones on main.
• Confirmed base/demos default branch is master and the template directory exists at that path.

How has it been tested?

• Verified the /tree/main/ URL returns 404 and the /tree/master/ URL returns 200.
• Swept the whole docs tree for base/demos/(blob|tree|raw)/main; these two links were the only matches.

[cb-heimdall]

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