Fix std err calculations

[guilhermebodin]

No description provided.

[codecov]

Codecov Report

Merging #305 (27d88c6) into master (6a825f7) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #305   +/-   ##
=======================================
  Coverage   93.04%   93.04%           
=======================================
  Files          35       35           
  Lines        3293     3293           
=======================================
  Hits         3064     3064           
  Misses        229      229           

Impacted Files	Coverage Δ
src/fit.jl	97.80% <100.00%> (ø)
src/models/common.jl	95.45% <100.00%> (ø)
src/models/naive_models.jl	100.00% <100.00%> (ø)
src/visualization/forecast.jl	92.00% <100.00%> (ø)

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