Update JuMP syntax in syntax.rst

[odow]

I saw @pz-max's comment in https://github.com/PyPSA/linopy/issues/271.

This PR just updates to match the benchmark implementation: https://github.com/PyPSA/linopy/blob/5d61f958a9b805d4c41ecb606a223d1125fa67c0/benchmark/scripts/benchmark_jump.jl#L9-L18

It's fair that most people probably write out the scalar I and j version though, so up to you whether you want to merge this.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.80%. Comparing base (5d61f95) to head (fb86bc0).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #272 +/- ## ========================================== - Coverage 88.94% 85.80% -3.14% ========================================== Files 15 15 Lines 3663 3663 Branches 861 861 ========================================== - Hits 3258 3143 -115 - Misses 281 405 +124 + Partials 124 115 -9 ```

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