Replace unicode symbol by ascii

MSVC was emitting warning C4819 before this commit.

include/libtorrent/kademlia/dht_storage.hpp

@@ -208,7 +208,7 @@ namespace libtorrent { namespace dht {
 		// This function retrieves a sample info-hashes
 		//
 		// For implementers:
-		// The info-hashes should be stored in ["samples"] (N × 20 bytes).
+		// The info-hashes should be stored in ["samples"] (N x 20 bytes).
 		// the following keys should be filled
 		// item["interval"] - the subset refresh interval in seconds.
 		// item["num"] - number of info-hashes in storage.