[word_count] Increase size of word_count input

exp/run.d/word_count.sh

@@ -10,7 +10,7 @@ prepare_word_count() {
   fi
 
   if [ ! -f $WORD_COUNT_INPUTS_DIR/input ]; then
-      for i in $(seq 1 10); do
+      for i in $(seq 1 15); do
           cat $WORD_COUNT_INPUTS_DIR/word_100MB.txt >> $WORD_COUNT_INPUTS_DIR/input
       done
   fi