Fix a-vs-an typos

test/functional/p2p_fingerprint.py

@@ -4,7 +4,7 @@
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 """Test various fingerprinting protections.
 
-If an stale block more than a month old or its header are requested by a peer,
+If a stale block more than a month old or its header are requested by a peer,
 the node should pretend that it does not have it to avoid fingerprinting.
 """