Update JuMP syntax in syntax.rst

[odow]

I saw @pz-max's comment in #271.

This PR just updates to match the benchmark implementation:

linopy/benchmark/scripts/benchmark_jump.jl

Lines 9 to 18 in 5d61f95

	function basic_model(n, solver)
	m = Model(solver)
	@variable(m, x[1:n, 1:n])
	@variable(m, y[1:n, 1:n])
	@constraint(m, x - y .>= 0:(n-1))
	@constraint(m, x + y .>= 0)
	@objective(m, Min, 2 * sum(x) + sum(y))
	optimize!(m)
	return objective_value(m)
	end

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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[FabianHofmann]

Great, thank you