fix grep parameter (shell not bash)

build.sh

@@ -6,7 +6,7 @@ if [[ $# -ne 1 ]]; then
 fi
 
 revision="$1"
-version=$(echo $revision | grep -Po 'v[0-9.]*')
+version=$(echo $revision | grep -Eo 'v[0-9.]*')
 commit_date=$(git show -s --format=%ci ${revision} | cut -c1-10)
 
 asciidoctor-pdf -a revnumber=${version} -a revdate=${commit_date} -o angelguide_en.pdf guide.adoc