Add capacitor initial voltage to README.md example

[jaakkor2]

Unless initial condition for capacitor initial voltage is given, ODEProblem errors

┌ Warning: Initialization system is underdetermined. 0 equations for 1 unknowns. Initialization will default to using least squares. To suppress this warning pass warninitializedetermined = false. To make this
│ warning into an error, pass fully_determined = true
└ @ ModelingToolkit C:\Users\jaakkor2\.julia\packages\ModelingToolkit\zfOUk\src\systems\diffeqs\abstractodesystem.jl:1294
┌ Warning: Did not converge after maxiters = 0 substitutions. Either there is a cycle in the rules or maxiters needs to be higher.
└ @ Symbolics C:\Users\jaakkor2\.julia\packages\Symbolics\8MbnV\src\variable.jl:546
ERROR: Found symbolic value -capacitor₊p₊v(t) for variable resistor₊p₊i(t). You may be missing an initial condition or have cyclic initial conditions. If this is intended, pass `symbolic_u0 = true`. In case the initial conditions are not cyclic but require more substitutions to resolve, increase `substitution_limit`. To report cycles in initial conditions of unknowns/parameters, pass `warn_cyclic_dependency = true`. If the cycles are still not reported, you may need to pass a larger value for `circular_dependency_max_cycle_length` or `circular_dependency_max_cycles`.

Initial condition v=0.0 is given here
https://github.com/SciML/ModelingToolkitStandardLibrary.jl/blob/v2.17.0/docs/src/tutorials/rc_circuit.md#L23 and here
https://github.com/SciML/ModelingToolkitStandardLibrary.jl/blob/v2.17.0/test/Electrical/analog.jl#L91

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.