Corrected config-env link to be URL (#1334)

The old relative link of `/docs/functions/config-env` doesn't work outside of this repo (for example when using this project as a dependency).
Also, markdown syntax didn't seem to be properly rendered on the [official API documentation](https://firebase.google.com/docs/reference/functions/firebase-functions).

Co-authored-by: Daniel Lee <danielylee@google.com>

src/v1/config.ts

@@ -37,7 +37,7 @@ export function resetCache(): void {
  * Store and retrieve project configuration data such as third-party API
  * keys or other settings. You can set configuration values using the
  * Firebase CLI as described in
- * [Environment Configuration](/docs/functions/config-env).
+ * https://firebase.google.com/docs/functions/config-env.
  */
 export function config(): Record<string, any> {
   // K_CONFIGURATION is only set in GCFv2