pptx: Fix capitalisation of notesMasterId

I don’t think this has caused any problems, but before now it’s been
"NotesMasterId", which is incorrect according to [ECMA-376].

[ECMA-376]: https://www.ecma-international.org/publications-and-standards/standards/ecma-376/

src/Text/Pandoc/Writers/Powerpoint/Output.hs

@@ -1981,7 +1981,7 @@ presentationToPresentationElement pres@(Presentation _ slds) = do
 
       notesMasterElem =  mknode "p:notesMasterIdLst" []
                          [ mknode
-                           "p:NotesMasterId"
+                           "p:notesMasterId"
                            [("r:id", "rId" <> tshow notesMasterRId)]
                            ()
                          ]