README.md: fix index.json description
README.md: fix index.json description

The index.json file maps module paths to timestamps, not package names.

This is now fixed in the README.md.

Change-Id: I217a45b51c78acdba6cdc07d4d5eb04d6a623781
Reviewed-on: https://go-review.googlesource.com/c/vulndb/+/342893
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Roland Shoemaker <0d270388f2f92757a5de0f4bd891d3b392c44c4f@golang.org>

golang/vuln,golang/vuln