Fix the incorrect Affine - Projective implementation

[winderica]

Description

This PR fixes the incorrect implementation of subtraction between an affine point and a projective point by flipping the position of subtrahend and minuend.

closes: #821

No performance regression due to this PR is observed (tested on Intel Core i9-12900k):

Arithmetic for Bls12_381::G1/Subtraction
                        time:   [443.13 ns 445.52 ns 448.43 ns]
                        change: [-0.7657% -0.0664% +0.5887%] (p = 0.86 > 0.05)
                        No change in performance detected.
Found 8 outliers among 100 measurements (8.00%)
  4 (4.00%) high mild
  4 (4.00%) high severe

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (master)
• Linked to GitHub issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code (N/A)
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the GitHub PR explorer

[Pratyush]

Suggested change

	self.into_group() - other
	self + (-other)

[winderica]

Thanks for pointing out the better solution! Wasn't aware that we can do this 😂

[Pratyush]

Thank you for the PR and the fix!