[Cirq-FT] Fix typo and add more helpful assertion message.

[fdmalone]

I often hit this assertion and have to poke around to figure out which register is missing. pe -> be.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01% ⚠️

Comparison is base (91f690d) 97.36% compared to head (510cde3) 97.36%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6221      +/-   ##
==========================================
- Coverage   97.36%   97.36%   -0.01%     
==========================================
  Files        1116     1116              
  Lines       95700    95700              
==========================================
- Hits        93178    93177       -1     
- Misses       2522     2523       +1     

Files Changed	Coverage Δ
cirq-core/cirq/testing/consistent_qasm_test.py	90.24% <100.00%> (ø)
cirq-ft/cirq_ft/infra/gate_with_registers.py	95.51% <100.00%> (ø)

... and 5 files with indirect coverage changes

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