Update Gtk doc link

[inkydragon]

old link 404: https://developer.gnome.org/gtk/stable

[tknopp]

can you make it a link again?

[giordano]

It was a link already, wasn't it? https://github.com/JuliaGraphics/Gtk.jl/blob/a5b92d960112bebfdf26f4c7815ca89a3b2ec411/README.md

[codecov]

Codecov Report

Merging #614 (1ddc9e3) into master (ce3b9de) will not change coverage.
The diff coverage is n/a.

❗ Current head 1ddc9e3 differs from pull request most recent head 6705fed. Consider uploading reports for the commit 6705fed to get more accurate results

@@           Coverage Diff           @@
##           master     #614   +/-   ##
=======================================
  Coverage   57.73%   57.73%           
=======================================
  Files          32       32           
  Lines        2721     2721           
=======================================
  Hits         1571     1571           
  Misses       1150     1150           

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[tknopp]

https://github.com/JuliaGraphics/Gtk.jl/blob/a5b92d960112bebfdf26f4c7815ca89a3b2ec411/README.md

then this is some Github rendering thing. Classic markdown will not generate a link.