fix format error

[rathann]

Fixes:

./tgz_test.go:87: Fatalf format %s reads arg #2, but call has only 1 arg

[codecov-io]

Codecov Report

Merging #3 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master       #3   +/-   ##
=======================================
  Coverage   74.13%   74.13%           
=======================================
  Files           1        1           
  Lines          58       58           
=======================================
  Hits           43       43           
  Misses          9        9           
  Partials        6        6

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