Fix the reference length

[pepicrft]

Short description 📝

This commit introduced a regression that caused the tests to fail because the generated object reference is 25 characters long instead of 24.

This PR fixes it.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@f87362c). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #477   +/-   ##
=========================================
  Coverage          ?   82.53%           
=========================================
  Files             ?      148           
  Lines             ?     8043           
  Branches          ?        0           
=========================================
  Hits              ?     6638           
  Misses            ?     1405           
  Partials          ?        0

Impacted Files	Coverage Δ
Sources/XcodeProj/Utils/ReferenceGenerator.swift	86.43% <100%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f87362c...e4d3334. Read the comment docs.

[giginet]

This issue caused by #474 😞

We should use ClosedRange instead of changing the test case.

String(reference[..<reference.index(reference.startIndex, offsetBy: 24)])

[pepicrft]

We should use ClosedRange instead of changing the test case.

Feel free to open a PR with that suggestion 🤗