BF: add missing f- for a string

[yarikoptic]

Ref: #1212

[codecov]

Codecov Report

Base: 89.16% // Head: 89.16% // No change to project coverage 👍

Coverage data is based on head (430800f) compared to base (f2e1672).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1228   +/-   ##
=======================================
  Coverage   89.16%   89.16%           
=======================================
  Files          74       74           
  Lines        9444     9444           
=======================================
  Hits         8421     8421           
  Misses       1023     1023           

Flag	Coverage Δ
unittests	89.16% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dandi/files/__init__.py	93.75% <ø> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[yarikoptic]

no brainer and green -- will just merge