doc: mark URL as such in Markdown

[fingolfin]

Unlike GitHub flavored Markdown, in "plain" Markdown URLs must be marked as such in order to be turned into links.

Right now it is not, see https://julialang.github.io/PackageCompiler.jl/dev/index.html#Installation-instructions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.16%. Comparing base (e87610c) to head (361d0a3).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1046   +/-   ##
=======================================
  Coverage   86.16%   86.16%           
=======================================
  Files           3        3           
  Lines         795      795           
=======================================
  Hits          685      685           
  Misses        110      110           

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