docs: correct onSuccess JSDoc + receivedRedirectUri type docs

[tsushanth]

Summary

Two small documentation fixes filed as separate issues but touching adjacent surfaces — combined into a single PR.

1. CommonPlaidLinkOptions.onSuccess JSDoc (closes #397)

src/types/index.ts:92-93 referenced public_key and had a connecter/connected typo. The first callback argument is public_token since the Link Token migration; public_key is the deprecated public-key auth flow and is no longer what consumers receive here. The matching PlaidLinkOnSuccess type already uses public_token.

-  // A function that is called when a user has successfully connecter an Item.
-  // The function should expect two arguments, the public_key and a metadata object
+  // A function that is called when a user has successfully connected an Item.
+  // The function should expect two arguments, the public_token and a metadata object
   onSuccess: T;

The other surviving public_key references in this file are inside the PlaidLinkOptionsWithPublicKey interface, which is the deprecated public-key flow — those are intentional and left untouched.

2. receivedRedirectUri README/type mismatch (closes #398)

README.md:87 documented receivedRedirectUri as string | null | undefined, but the type at src/types/index.ts:156 only permits string | undefined. Updated the README to match the type — that's the minimal, lower-risk path; widening the type would change the runtime contract.

-| `receivedRedirectUri` | `string \| null \| undefined`                                                             |
+| `receivedRedirectUri` | `string \| undefined`                                                                     |

Test plan

• Comment-only / docs-only change — no runtime behavior change
• PlaidLinkOnSuccess type already uses public_token, so JSDoc now matches the type
• receivedRedirectUri type unchanged; README now matches it