Fix Blom 1958 reference

[ahartikainen]

This PR fixes the reference.

This came up in arviz-devs/arviz#1366 (comment)

Thanks @avehtari for the correction.

[codecov-commenter]

Codecov Report

Merging #86 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #86   +/-   ##
=======================================
  Coverage   92.81%   92.81%           
=======================================
  Files          24       24           
  Lines        1447     1447           
=======================================
  Hits         1343     1343           
  Misses        104      104           

Impacted Files	Coverage Δ
R/convergence.R	88.75% <ø> (ø)

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[paul-buerkner]

Thanks!