fix warning when if css_html_js_minify is not installed

[cserteGT3]

I got the following warning message when running publish():

┌ Warning: I didn't find css_html_js_minify, you can install it via pip the output will not be minified.
└ @ JuDoc C:\Users\Laci\.julia\packages\JuDoc\AAWqV\src\manager\post_processing.jl:57

I assume, that a period is missing, so fixed it.

[codecov]

Codecov Report

Merging #192 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #192   +/-   ##
=======================================
  Coverage   87.12%   87.12%           
=======================================
  Files          26       26           
  Lines        1080     1080           
=======================================
  Hits          941      941           
  Misses        139      139

Impacted Files	Coverage Δ
src/manager/post_processing.jl	26.66% <0%> (ø)	⬆️

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