Fix example code for EditorImportPlugin

doc/classes/EditorImportPlugin.xml

@@ -39,8 +39,8 @@
 		    return [{"name": "my_option", "default_value": false}]
 
 		func load(src, dst, opts, r_platform_variants, r_gen_files):
-		    var f = File.new()
-		    if f.open(src, File.READ) != OK:
+		    var file = File.new()
+		    if file.open(src, File.READ) != OK:
 		        return FAILED
 
 		    var mesh = Mesh.new()