[doc] fix invalid env vars

[stas00]

• remove invalid ENV_ prefix.
• add a few : while at it

@sgugger

[codecov]

Codecov Report

Merging #6504 into master will decrease coverage by 2.17%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #6504      +/-   ##
==========================================
- Coverage   80.38%   78.20%   -2.18%     
==========================================
  Files         156      156              
  Lines       28058    28058              
==========================================
- Hits        22554    21943     -611     
- Misses       5504     6115     +611     

Impacted Files	Coverage Δ
src/transformers/trainer.py	37.84% <0.00%> (ø)
src/transformers/optimization.py	25.55% <0.00%> (-70.00%)	⬇️
src/transformers/pipelines.py	26.26% <0.00%> (-53.69%)	⬇️
src/transformers/optimization_tf.py	33.33% <0.00%> (-24.33%)	⬇️
src/transformers/modeling_tf_auto.py	48.79% <0.00%> (-18.08%)	⬇️
src/transformers/data/processors/squad.py	13.76% <0.00%> (-14.38%)	⬇️
src/transformers/modeling_auto.py	64.36% <0.00%> (-14.37%)	⬇️
src/transformers/modeling_tf_gpt2.py	65.68% <0.00%> (-6.16%)	⬇️
src/transformers/modeling_tf_bert.py	66.00% <0.00%> (-3.06%)	⬇️
src/transformers/modelcard.py	82.71% <0.00%> (-2.47%)	⬇️
... and 9 more

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[JetRunner]

Great!