doc: Fix error in QOpenGLShaderProgram example

The addShader() function takes a pointer.

Change-Id: Ibd6549ff06cb3bd86096e90ab2637845f032fc68
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>

src/gui/doc/snippets/code/src_gui_qopenglshaderprogram.cpp

@@ -43,7 +43,7 @@ QOpenGLShader shader(QOpenGLShader::Vertex);
 shader.compileSourceCode(code);
 
 QOpenGLShaderProgram program(context);
-program.addShader(shader);
+program.addShader(&shader);
 program.link();
 
 program.bind();