Fix typos

Found via  
```
codespell -q 3 -L aci,ake,aline,alle,alledges,alocation,als,ang,anid,ba,beginn,behaviour,bloaded,byteorder,calculater,cancelled,cancelling,cas,cascade,centimetre,childs,colour,colours,commen,connexion,currenty,dof,doubleclick,dum,eiter,elemente,ende,feld,finde,findf,freez,hist,iff,indicies,initialisation,initialise,initialised,initialises,initialisiert,ist,kilometre,lod,mantatory,methode,metres,millimetre,modell,nd,noe,normale,normaly,nto,numer,oder,orgin,orginx,orginy,ot,pard,pres,programm,que,recurrance,rougly,seperator,serie,sinc,strack,substraction,te,thist,thru,tread,uint,unter,vertexes,wallthickness,whitespaces -S ./.git,*.po,*.ts,./ChangeLog.txt,./src/3rdParty,./src/Mod/Assembly/App/opendcm,./src/CXX,./src/zipios++,./src/Base/swig*,./src/Mod/Robot/App/kdl_cp,./src/Mod/Import/App/SCL,./src/WindowsInstaller,./src/Doc/FreeCAD.uml
```

src/App/Application.cpp

@@ -707,8 +707,8 @@ std::vector<Document*> Application::openDocuments(const std::vector<std::string>
     std::vector<Document*> docs;
     docs.reserve(newDocs.size());
     for (auto &v : newDocs) {
-        // Notify ProeprtyXLink to attach newly opened documents and restore
-        // relavant external links
+        // Notify PropertyXLink to attach newly opened documents and restore
+        // relevant external links
         PropertyXLink::restoreDocument(*v.first);
         docs.push_back(v.first);
     }