[Longformer] fix model name in examples

[ibeltagy]

This PR fixes the model identifier of Longformer to the new standard fomat <organisation/model_name>

[codecov-commenter]

Codecov Report

Merging #4653 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4653   +/-   ##
=======================================
  Coverage   77.19%   77.20%           
=======================================
  Files         128      128           
  Lines       21021    21021           
=======================================
+ Hits        16228    16230    +2     
+ Misses       4793     4791    -2     

Impacted Files	Coverage Δ
src/transformers/modeling_longformer.py	96.82% <ø> (ø)
src/transformers/modeling_tf_utils.py	88.83% <0.00%> (+0.16%)	⬆️
src/transformers/file_utils.py	73.85% <0.00%> (+0.41%)	⬆️

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