Utils: Fix an occurrence of the old IPlugin::initialize() signature

... in a comment.

Change-Id: I9d3e3ce03bccd96b52c6276be83c202b069e71e1
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/libs/utils/macroexpander.cpp

@@ -232,7 +232,7 @@ using namespace Internal;
     A typical setup is to register variables in the Plugin::initialize() function.
 
     \code
-    bool MyPlugin::initialize(const QStringList &arguments, QString *errorString)
+    void MyPlugin::initialize()
     {
         [...]
         MacroExpander::registerVariable(