added a pull before the push

pdf/update_pdf.sh

@@ -1,6 +1,7 @@
 #! /bin/bash
 cd ..
 git checkout gh-pages
+git pull
 git checkout master Kalman_and_Bayesian_Filters_in_Python.pdf
 git checkout master README.md
 git add Kalman_and_Bayesian_Filters_in_Python.pdf