fix to_signed_felt doc

[greged93]

Fix doc tests

Description

The current documentation for to_signed_felt and take_until_unbalanced are incorrect and caused the doc tests to fail.

Checklist

• Linked to Github Issue
• Unit tests added
• Integration tests added.
• This change requires new documentation.
• Documentation has been added/updated.
• CHANGELOG has been updated.

[MegaRedHand]

Could you add an entry to the changelog regarding this change? Something like:

* fix: `take_until_unbalanced` and `to_signed_felt` doctests no longer fail [#1262](https://github.com/lambdaclass/cairo-rs/pull/1262)

[codecov]

Codecov Report

Merging #1262 (7abe5a0) into main (9cef244) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1262   +/-   ##
=======================================
  Coverage   97.60%   97.60%           
=======================================
  Files          89       89           
  Lines       36335    36335           
=======================================
  Hits        35466    35466           
  Misses        869      869           

Impacted Files	Coverage Δ
felt/src/lib.rs	90.68% <ø> (ø)

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[Oppen]

@MegaRedHand to_signed_felt will be removed after merging the standard felt, right? If that's so, @greged93 would you agree to closing this PR?

[MegaRedHand]

The function won't be removed in the foreseeable future. We can merge this for now (once we update the changelog).

Context: we are changing the underlying implementation of Felt252 for performance reasons, and some of its API might change. We still need to be able to treat felts as signed for some hints, so we still need this method.

[greged93]

Will rebase and update the change log then :)

[greged93]

I think the PR can be closed, doctests were fixed by #1281 :)