docs: Fix Typo Update CONTRIBUTING.md

[famouswizard]

What

There is one typo in text:

Correction:

In the "Making Changes" section:

• Set the stellar/stellar-cli repo as new remote upstream repository that will sync with your fork.

Instead of "new", it should be "the". The correct sentence:

• Set the stellar/stellar-cli repo as the remote upstream repository that will sync with your fork.

Why

• "new" is incorrect here because it refers to setting the standard remote repository, not a new one.