Update project name pattern in JSON schema

This update landed in compose-spec/compose-spec#314, reflecting the new
project name spec:

> Project name MUST contain only lowercase letters, decimal digits,
> dashes, and underscores, and MUST begin with a lowercase letter or
> decimal digit.

Signed-off-by: Mike Bland <[email protected]>

Author: mbland

schema/compose-spec.json

@@ -13,6 +13,7 @@
 
     "name": {
       "type": "string",
+      "pattern": "^[a-z0-9][a-z0-9_-]*$",
       "description": "define the Compose project name, until user defines one explicitly."
     },