Fix incorrect/misleading sentence in string interpolation section

[v4nft]

The sentence says we'll be constructing a string regarding a web server URL but the example code constructs an S3 URL from an S3 bucket and object instead. I changed the sentence to correctly describe the code example.

Description

Checklist:

• I have reviewed the style guide.
• If blogging, I have reviewed the blogging guide.
• I have manually confirmed that all new links work.
• I added aliases (i.e., redirects) for all filename changes.
• If making css changes, I rebuilt the bundle.