Fixed copy when directory not empty

Was copying the kf_book subdirectory into the existing one
if it was already there, causing PDF build failure if you
didn't clean prior to running it.

pdf/run_notebooks.bat

@@ -1,7 +1,7 @@
 mkdir tmp
 copy ..\*.ipynb .\tmp
 copy ..\*.py .\tmp
-cp -r ..\kf_book\ .\tmp\kf_book\
+cp -r ..\kf_book\ .\tmp\
 
 cd tmp