update outdated link to reentrancy explainer

[platonicsocrates]

Hey All! The link to the re-entrancy guide is out of date. It has now moved to the link updated in this PR.

Here's the note on the old page for reference:

Thank you for visiting the Smart Contract Security Best Practices. Please note that this resource is no longer actively maintained. Instead, we recommend visiting the Smart Contract Security Field Guide. The Field Guide is regularly updated and curated by the same security engineer who previously contributed to the Best Practices guide.

The resource on reentrancy can be found here: https://scsfg.io/hackers/reentrancy/