[ci skip] docs - typo in prescript

docs/pre_script.sh

 #!/bin/sh
+set -x
 echo "generating doxygen for Limbo, current path $PWD"
 doxygen Doxyfile
 
@@ -17,7 +18,7 @@ cp -r $BENCHMARKS/fig_benchmarks docs
 
 echo "getting the latest regression benchmark result (needs to be in $HOME/limbo_reg_benchmarks)"
 DIR=$HOME/limbo_reg_benchmarks
-BENCHMARKS=$DIR/`ls -t $DIR | |head -n 1`
+BENCHMARKS=$DIR/`ls -t $DIR | head -n 1`
 cp $BENCHMARKS/regression_benchmarks.rst docs/reg_benchmarks.rst
 cp -r $BENCHMARKS/regression_benchmarks_figs docs/fig_benchmarks