Fix AttributeError in HMC bad initial energy warning

[michaelosthege]

What is this PR about?
Fixing an AttributeError: 'numpy.ndarray' object has no attribute 'to_string'.

This is a bit strange, because as far as I can tell np.ndarray never had a .to_string() method to beging with.
There's a tostring() which got deprecated in NumPy 1.19 however.

In any case, the f-string should fix it.

Checklist

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Major / Breaking Changes

None

Bugfixes / New features

Fixes an AttributeError in the "Bad initial energy" warning.

Docs / Maintenance

None

[codecov]

Codecov Report

Merging #6037 (e89665c) into main (ad16bf4) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #6037      +/-   ##
==========================================
+ Coverage   89.27%   89.29%   +0.02%     
==========================================
  Files          72       72              
  Lines       12890    12890              
==========================================
+ Hits        11507    11510       +3     
+ Misses       1383     1380       -3     

Impacted Files	Coverage Δ
pymc/step_methods/hmc/base_hmc.py	90.55% <ø> (ø)
pymc/parallel_sampling.py	86.79% <0.00%> (+0.99%)	⬆️