Fix missing f in f-string

[paalbra]

Fixes: #429

Same request as in the issue now produces:

GET /servers/getPyMISPVersion.json HTTP/1.1
Host: localhost:1337
User-Agent: PyMISP 2.4.111.2 - Python 3.7
Accept-Encoding: gzip, deflate
Accept: application/json
Connection: keep-alive
Authorization: asdf
content-type: application/json

[Rafiot]

.... That's a stupid bug, thank you very much for finding and fixing it.

[codecov]

Codecov Report

Merging #430 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #430   +/-   ##
=======================================
  Coverage   39.94%   39.94%           
=======================================
  Files          34       34           
  Lines        5881     5881           
=======================================
  Hits         2349     2349           
  Misses       3532     3532

Impacted Files	Coverage Δ
pymisp/aping.py	12.92% <0%> (ø)	⬆️

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