Clarify Catalog Layout Best Practices for collections

Here's an attempt to simplify and clarify the best practices 
section for catalog layout to address radiantearth#925.

In practice, we've found that every Catalog is named catalog.json
and every Collection is named collection.json, which is enforced
by tools such as PySTAC.

best-practices.md

@@ -205,8 +205,8 @@ We encourage people to explore new structures of linking data, but the following
 ended up doing. Following these recommendations makes for more legible catalogs.
 
 1. Root documents (catalogs / collections) should be at the root of a directory tree containing the static catalog.
-2. Catalogs should be named `catalog.json` (cf. `index.html`).
-3. Collections that are distinct from catalogs should be named `collection.json`.
+2. Catalogs should be named `catalog.json` and Collections should be named `collection.json`.
+3. Collections at the root of a Catalog should be named `collection.json`.
 4. Items should be named `<id>.json`.
 5. Sub-catalogs should be stored in subdirectories of their parent (and only 1 subdirectory deeper than a document's parent) (e.g. `.../sample/sub1/catalog.json`).
 6. Items should be stored in subdirectories of their parent catalog.