Update protocol.rst

[JRodez]

typo in execute job specification

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Link to related issues if relevant. For example #42.

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Branch name.

• [ ] Descriptive and short
• [ ] Use hyphens to separate words

Branch content.

• [ ] Only dedicated to the problem.
• [ ] Based on Batsim's official master branch.
• [ ] Straightforward. Just a sequence of commits. Does not contain merge commits.
• [ ] Test results are not worse than before. How to run Batsim tests?

TL; DR : this is a doc fix