fix: missing :action tag

[marcofavorito]

Proposed changes

During the conversion from PDDL domain to string, the actions were printed without the :action tag. This PR adds the tag and removes the (wrong) upper-level tag :actions

Fixes

n/a

Types of changes

What types of changes does your code introduce?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

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• I have read the CONTRIBUTING doc
• I am making a pull request against the develop branch (left side). Also you should start your branch off our develop.
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works

Further comments

n/a

[codecov]

Codecov Report

Merging #27 (e1a948b) into develop (7c05194) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop      #27   +/-   ##
========================================
  Coverage    89.49%   89.49%           
========================================
  Files           19       19           
  Lines          790      790           
  Branches        71       71           
========================================
  Hits           707      707           
  Misses          64       64           
  Partials        19       19           

Flag	Coverage Δ
unittests	89.49% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pddl/formatter.py	100.00% <ø> (ø)
pddl/core.py	93.07% <100.00%> (ø)

[francescofuggitti]

Thanks!