doc: Fix grammatical errors in multisig-tutorial.md #31225

This pull request fixes grammatical errors in the 'multisig-tutorial.md' document.

Corrections:

1. Incorrect Phrase "As can been seen":

   - Before:

     There are discussions about eliminating this redundancy, as can been seen in the issue #17190 (https://github.com/bitcoin/bitcoin/issues/17190).

   - After:

     There are discussions about eliminating this redundancy, as can be seen in the issue #17190 (https://github.com/bitcoin/bitcoin/issues/17190).

2. Clarity Improvement in a Sentence:

   - Before:

     Note that at least two descriptors are usually used, one for internal derivation paths and external ones.

   - After:

     Note that at least two descriptors are usually used, one for internal derivation paths and one for external ones.

Explanation:

- Corrected "been" to "be" to fix the grammatical error.
- Added "one for" before "external ones" to improve clarity and parallel structure in the sentence.

These minor corrections enhance the readability and professionalism of the documentation. Thank you for considering this pull request.