docs: polish algorithms tutorial

[Gaiejj]

Description

• Unify symbol system
• FIx typo
• Polish statement

I have built the docs on https://omnisafe-zjy.readthedocs.io/en/latest/

Types of changes

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

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist

Go over all the following points, and put an x in all the boxes that apply.
If you are unsure about any of these, don't hesitate to ask. We are here to help!

• I have read the CONTRIBUTION guide. (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly. (required for a bug fix or a new feature)
• I have updated the documentation accordingly.
• I have reformatted the code using make format. (required)
• I have checked the code using make lint. (required)
• I have ensured make test pass. (required)

[codecov-commenter]

Codecov Report

Merging #242 (685a226) into main (8bdcd43) will increase coverage by 3.43%.
The diff coverage is 78.79%.

❗ Current head 685a226 differs from pull request most recent head 776d6be. Consider uploading reports for the commit 776d6be to get more accurate results

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@@            Coverage Diff             @@
##             main     #242      +/-   ##
==========================================
+ Coverage   93.53%   96.96%   +3.43%     
==========================================
  Files         134      134              
  Lines        6905     6870      -35     
==========================================
+ Hits         6458     6661     +203     
+ Misses        447      209     -238     

Impacted Files	Coverage Δ
omnisafe/adapter/modelbased_adapter.py	94.12% <ø> (+0.73%)	⬆️
omnisafe/algorithms/model_based/base/loop.py	97.87% <ø> (+4.77%)	⬆️
omnisafe/algorithms/model_based/planner/arc.py	100.00% <ø> (+2.17%)	⬆️
omnisafe/algorithms/model_based/planner/cem.py	100.00% <ø> (+2.22%)	⬆️
omnisafe/algorithms/off_policy/sac.py	100.00% <ø> (ø)
omnisafe/algorithms/off_policy/td3.py	100.00% <ø> (ø)
omnisafe/algorithms/on_policy/base/natural_pg.py	100.00% <ø> (ø)
...isafe/algorithms/on_policy/base/policy_gradient.py	100.00% <ø> (ø)
omnisafe/algorithms/on_policy/base/ppo.py	100.00% <ø> (ø)
omnisafe/algorithms/on_policy/base/trpo.py	90.14% <ø> (ø)
... and 39 more

... and 3 files with indirect coverage changes

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[muchvo]

LGTM.