Fix missing backslash in latex for readme

[meliache]

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Description

While checking the available methods in the github readme I noticed that one $Delta$ didn't render properly as $\Delta$ so I decided to quickly fix add the missing backslash. On my fork this renders properly now. For testing I just ran pre-commit to check that I my editor didn't do any weird whitespace changes that I didn't notice. But I didn't run the unit tests or the doctests as I didn't change anything in the code. Should I?

Checklist

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't any other open Pull Request for the required change ?
• Does your submission pass pre-commit? ($ pre-commit run --all-files or $ nox -s lint)
• Does your submission pass tests? ($ pytest or $ nox -s tests)
• Does the documentation build with your changes? ($ cd docs; make clean; make html or $ nox -s docs)
• Does your submission pass doctests? ($ xdoctest ./src/vector or $ nox -s doctests)

Before Merging

• Summarize commit messages into a brief review of the Pull request.

[codecov-commenter]

Codecov Report

Base: 81.08% // Head: 81.08% // No change to project coverage 👍

Coverage data is based on head (f465e86) compared to base (4fbbd81).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #285   +/-   ##
=======================================
  Coverage   81.08%   81.08%           
=======================================
  Files          96       96           
  Lines       10672    10672           
=======================================
  Hits         8653     8653           
  Misses       2019     2019           

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[Saransh-cpp]

Thanks, @meliache! No need to run the tests locally :)