[!830] correct name PTScotch_FOUND variable in AddPTScotchFlags

Merge branch 'feature/find_ptscotch' into 'master'

ref:core/dune-common\>

### Summary

Quick-Fix of a typo in the cmake variable name

See merge request [core/dune-common!830]

  [core/dune-common!830]: gitlab.dune-project.org/core/dune-common/merge_requests/830

cmake/modules/AddPTScotchFlags.cmake

@@ -19,7 +19,7 @@ if(PTScotch_FOUND)
 endif()
 
 function(add_dune_ptscotch_flags _targets)
-  if(PTSCOTCH_FOUND)
+  if(PTScotch_FOUND)
     foreach(_target ${_targets})
       target_link_libraries(${_target} PTScotch::PTScotch)
     endforeach(_target ${_targets})