Update command line in plugins readme

As noted in #476, the command line option to enable plugins was out of
date. The option was changed in
2e2fca0. This commit updates the readme
so that it contains the correct command.

Plugins/Readme.txt

@@ -5,4 +5,4 @@ To load the plugins here, go through the following steps:
 1. Copy the desired plugins into a "plugins" folder in the same directory as the Explorer++ executable.
 2. Start Explorer++ with plugin support enabled:
 
-explorer++.exe --enable-plugins
+explorer++.exe --enable-features Plugins