update Ubuntu and Debian install instructions in README.md

[wk]

Pre-Request Checklist

• Passes rubocop code analysis (try rubocop --auto-correct)
• Tests added or adapted (try rake test)
• Changes are reflected in the documentation
• User-visible changes appended to CHANGELOG.md

Description

Minor documentation update.

[codecov-io]

Codecov Report

Merging #1591 into master will decrease coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1591      +/-   ##
==========================================
- Coverage   62.54%   62.38%   -0.16%     
==========================================
  Files          30       30              
  Lines        1495     1494       -1     
==========================================
- Hits          935      932       -3     
- Misses        560      562       +2

Impacted Files	Coverage Δ
lib/oxidized/output/git.rb	16.4% <0%> (-1.57%)	⬇️
lib/oxidized/node.rb	71.32% <0%> (-0.2%)	⬇️

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