Indent externals sub properties in the README

[JamesLMilner]

Type: bug

The following has been addressed in the PR:

• There is a related issue
• All code has been formatted with prettier
• Unit or Functional tests are included in the PR

Description:

At the moment these properties are at the same level as the externals property, when they should be nested under dependencies

[codecov]

Codecov Report

Merging #205 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #205   +/-   ##
=======================================
  Coverage   52.88%   52.88%           
=======================================
  Files          10       10           
  Lines         571      571           
  Branches      139      139           
=======================================
  Hits          302      302           
  Misses        269      269

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