Reduce concurrent requests

Countdown API returns 500 Internal Server Errors when attempting to
request too many pages at once. We mitigate this by reducing the
concurrent threads from 12 to 6.

src/lib.rs

@@ -12,4 +12,4 @@ pub const CACHE_PATH: &str = "cache.json";
 /// The amount of milliseconds to wait between performing iterations on the pages.
 pub const PAGE_ITERATION_INTERVAL: Duration = Duration::from_millis(500);
 /// The amount of requests to perform in parallel.
-pub const CONCURRENT_REQUESTS: i64 = 12;
+pub const CONCURRENT_REQUESTS: i64 = 6;