Pcbnew: change step export dialog action button text.

Change "OK" to "Export" so user is not surprised by the fact that the
dialog does not close after the step file is created..

Fixes lp:1784260

https://bugs.launchpad.net/kicad/+bug/1784260

pcbnew/dialogs/dialog_export_step.cpp

@@ -120,6 +120,7 @@ DIALOG_EXPORT_STEP::DIALOG_EXPORT_STEP( PCB_EDIT_FRAME* aParent, const wxString&
     m_boardPath = aBoardPath;
     m_config = Kiface().KifaceSettings();
     m_sdbSizerCancel->SetLabel( _( "Close" ) );
+    m_sdbSizerOK->SetLabel( _( "Export" ) );
     m_sdbSizer->Layout();
 
     // Build default output file name