doc: Fix PR reference in productivity guide #36211

Notice that the refspec in doc/productivity.md maps to upstream-pull/NUMBER, but the doc mentions upstream-pull/NUMBER/head right below it. Dropping the trailing /head so the git show / git chekcout examples actually match up. Tested both locally in a scratch repo and they work fine now.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36211.

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<details> <summary>Pull request overview</summary>

Updates the productivity guide to match the configured Git refspec.

Changes:

• Removes the incorrect trailing /head from the PR reference.

  </details>

<details> <summary>File summaries</summary>

</details>

<details> <summary>Review details</summary>

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

  </details>

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ACK b3a9b84b37c0c30f4411ad1e09d8f89e9aaa7434