Update formal verification section in README.md

[pennyannn]

Description of changes:

This PR updates the formal verification section in README with P-256 curve.

The update could be confirmed by checking:
The third_party/s2n-bignum directory (https://github.com/aws/aws-lc/tree/main/third_party/s2n-bignum) against the places the functions are actually used (https://github.com/aws/aws-lc/blob/main/crypto/fipsmodule/ec/p384.c for P384, https://github.com/aws/aws-lc/blob/main/crypto/fipsmodule/ec/p521.c for P512, https://github.com/aws/aws-lc/blob/main/crypto/fipsmodule/ec/p256-nistz.c for P256, https://github.com/aws/aws-lc/blob/main/crypto/fipsmodule/curve25519/curve25519_s2n_bignum_asm.c for X25519 and ED25519, and PR #1135 for RSA).

Call-outs:

Point out areas that need special attention or support during the review process. Discuss architecture or design changes.

Testing:

How is this change tested (unit tests, fuzz tests, etc.)? Are there any testing steps to be verified by the reviewer?

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.72%. Comparing base (93c213f) to head (8048a71).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2301      +/-   ##
==========================================
- Coverage   78.74%   78.72%   -0.02%     
==========================================
  Files         616      616              
  Lines      107588   107588              
  Branches    15286    15286              
==========================================
- Hits        84715    84698      -17     
- Misses      22218    22236      +18     
+ Partials      655      654       -1     

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