[tr] Polish translation update: don't redirect to non-existing Polish…

… documentation

i18n/qgis_pl.ts

         <location filename="../src/app/qgisapp.cpp" line="9079"/>
         <source>en</source>
         <comment>documentation language</comment>
-        <translation>pl</translation>
+        <translation>en</translation>
     </message>
     <message>
         <location filename="../src/app/qgisapp.cpp" line="9101"/>