Bugfix in _add_distributed_constraint

[JordiManyer]

While porting to Gridap v0.20, I did not contemplate the case where the FESpace has multiple reffes. See gridap/GridapEmbedded.jl#120

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.56%. Comparing base (cbd7197) to head (fb5aa1e).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
src/FESpaces.jl	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #201      +/-   ##
==========================================
- Coverage   84.68%   84.56%   -0.12%     
==========================================
  Files          16       16              
  Lines        4237     4238       +1     
==========================================
- Hits         3588     3584       -4     
- Misses        649      654       +5     

Flag	Coverage Δ
mpi-adaptivity	45.76% <12.50%> (-0.23%)	⬇️
mpi-fespaces	40.21% <77.77%> (-0.11%)	⬇️
mpi-geometry	23.36% <12.50%> (-0.24%)	⬇️
mpi-misc	50.33% <77.77%> (-0.11%)	⬇️
mpi-physics	35.50% <12.50%> (-0.23%)	⬇️
mpi-transient	25.01% <12.50%> (-0.24%)	⬇️
seq-adaptivity	39.66% <12.50%> (-0.23%)	⬇️
seq-fespaces	41.91% <77.77%> (-0.11%)	⬇️
seq-geometry	23.40% <12.50%> (-0.24%)	⬇️
seq-misc	35.11% <12.50%> (-0.23%)	⬇️
seq-physics	35.50% <12.50%> (-0.23%)	⬇️
seq-transient	24.24% <12.50%> (-0.26%)	⬇️

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