Fix typo in SaveFileTask error log message

Corrected "filepath" to "file path" in the log message to improve readability and maintain consistency in error reporting.

src/main/java/de/danielluedecke/zettelkasten/tasks/SaveFileTask.java

@@ -119,7 +119,7 @@ protected Object doInBackground() {
 		File filepath = settingsObj.getMainDataFile();
 		if (filepath == null) {
 			saveOk = false;
-			Constants.zknlogger.log(Level.SEVERE, "Failed to save main data file: filepath is null.");
+			Constants.zknlogger.log(Level.SEVERE, "Failed to save main data file: file path is null.");
 			return null;
 		}