[ElectraForQuestionAnswering] fix qa example in doc

[patil-suraj]

This PR fixes the QA example in the doc for ElectraForQuestionAnswering. In the last example token_type_ids were not used, this PR fixes that

@sgugger

[codecov]

Codecov Report

Merging #4929 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4929   +/-   ##
=======================================
  Coverage   77.10%   77.11%           
=======================================
  Files         128      128           
  Lines       21723    21723           
=======================================
+ Hits        16749    16751    +2     
+ Misses       4974     4972    -2     

Impacted Files	Coverage Δ
src/transformers/modeling_electra.py	78.16% <ø> (ø)
src/transformers/file_utils.py	73.38% <0.00%> (-0.41%)	⬇️
src/transformers/modeling_utils.py	90.61% <0.00%> (+0.11%)	⬆️
src/transformers/modeling_transfo_xl.py	78.81% <0.00%> (+0.19%)	⬆️
src/transformers/trainer.py	39.48% <0.00%> (+0.23%)	⬆️

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[sgugger]

LGTM

[LysandreJik]

LGTM