Fix typo

Author: veeck

module-development/core-module-file.md

@@ -283,7 +283,7 @@ getTemplateData: function() {
 Whenever the MagicMirror needs to update the information on screen (because it
 starts, or because your module asked a refresh using `this.updateDom()`), the
 system calls the getHeader method to retrieve the module's header. This method
-should therefor return a string. If this method is not subclassed, this function
+should therefore return a string. If this method is not subclassed, this function
 will return the user's configured header.
 
 If you want to use the original user's configured header, reference