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 <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
diff --git a/README.md b/README.md
index e411786..4c4b875 100644
--- a/README.md
+++ b/README.md
@@ -16,8 +16,8 @@
 
 https://storage.googleapis.com/go-vulndb/
 
-An `index.json` file maps package names to last modified timestamps. For each
-package name, a `NAME.json` file contains a list of vulnerability entries.
+An `index.json` file maps module paths to last modified timestamps. For each
+module, a `NAME.json` file contains a list of vulnerability entries.
 
 Note that this path and format are provisional and likely to change until an
 approved proposal.