Fix autoplot missing name error (and previous warning)

[cregouby]

• Fix autoplot(model_fit) fails with Can't unnest elements with missing names. #112
• remove duplicate code chunk for transpose_metrics() and add a test for it.
• improve README
• fix documentation typos

[dfalbel]

Looks good to me!

[codecov]

Codecov Report

Merging #113 (857ed13) into main (5f5a702) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #113      +/-   ##
==========================================
- Coverage   87.45%   87.39%   -0.06%     
==========================================
  Files          10       10              
  Lines        1060     1063       +3     
==========================================
+ Hits          927      929       +2     
- Misses        133      134       +1     

Impacted Files	Coverage Δ
R/model.R	94.59% <100.00%> (ø)
R/pretraining.R	94.30% <100.00%> (ø)
R/package.R	0.00% <0.00%> (ø)
R/parsnip.R	99.08% <0.00%> (+<0.01%)	⬆️
R/explain.R	92.50% <0.00%> (+0.19%)	⬆️

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