docs: Add concrete bundle prefix example to build-toolchain in README

[joshuacleetus]

Description
The README uses placeholder $BUNDLE_PREFIX without a real example.

Current:

$ ./swift/utils/build-toolchain $BUNDLE_PREFIX

Proposed:

$ ./swift/utils/build-toolchain com.example.my-toolchain


Result:

• Bundle identifier: com.example.my-toolchain.YYYYMMDD
• Output archive: swift-LOCAL-YYYY-MM-DD-a-osx.tar.gz

The resulting .tar.gz file will be created in the current working directory.


Motivation:
Lowers barrier for first-time builders.


Testing:
N/A (documentation only).

Checklist:
- [x] Follows Contributing Guide

Suggested label: documentation

[al45tair]

@joshuacleetus I think this is the kind of suggestion where the best thing would be for you to raise a PR with your suggested changes.

[joshuacleetus]

@al45tair Thanks! I was already planning to make one I will raise the PR soon.