Fix typos, grammar, and some whitespace issues

Found via `codespell -q 2 -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,currenty,dof,doubleclick,dum,eiter,elemente,feld,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/Gui/ExpressionCompleter.cpp

@@ -439,8 +439,8 @@ void ExpressionCompleter::slotUpdate(const QString & prefix, int pos)
     // in the middle of a token, and we shall include that token.
     for(auto it=tokens.begin();it!=tokens.end();++it) {
         if(get<1>(*it) >= pos) {
-            // Include the immediatly followed '.' or '#', because we'll be
-            // inserting these separater too, in ExpressionCompleteModel::pathFromIndex()
+            // Include the immediately followed '.' or '#', because we'll be
+            // inserting these separators too, in ExpressionCompleteModel::pathFromIndex()
             if(it!=tokens.begin() && get<0>(*it)!='.' && get<0>(*it)!='#')
                 it = it-1;
             tokens.resize(it-tokens.begin()+1);