eth/downloader: minor typo fixes in comments (#21035)

eth/downloader/downloader_test.go

@@ -1126,7 +1126,7 @@ func testBlockHeaderAttackerDropping(t *testing.T, protocol int) {
 	chain := testChainBase.shorten(1)
 
 	for i, tt := range tests {
-		// Register a new peer and ensure it's presence
+		// Register a new peer and ensure its presence
 		id := fmt.Sprintf("test %d", i)
 		if err := tester.newPeer(id, protocol, chain); err != nil {
 			t.Fatalf("test %d: failed to register new peer: %v", i, err)