Fix syntax error in new exceptions

Bug from https://github.com/FreshRSS/FreshRSS/pull/1261

app/Exceptions/AlreadySubscribedException.php

@@ -5,10 +5,10 @@ class FreshRSS_AlreadySubscribed_Exception extends Exception {
 
 	public function __construct($url, $feedName) {
 		parent::__construct('Already subscribed! ' . $url, 2135);
-		$this->$feedName = $feedName;
+		$this->feedName = $feedName;
 	}
 
 	public function feedName() {
-		return $this->feedName();
+		return $this->feedName;
 	}
 }

app/Exceptions/FeedNotAddedException.php

@@ -5,10 +5,10 @@ class FreshRSS_FeedNotAdded_Exception extends Exception {
 
 	public function __construct($url, $feedName) {
 		parent::__construct('Feed not added! ' . $url, 2147);
-		$this->$feedName = $feedName;
+		$this->feedName = $feedName;
 	}
 
 	public function feedName() {
-		return $this->feedName();
+		return $this->feedName;
 	}
 }