fix: make update script compatible with both MacOS and Linux

Fix sed inplace option to run with both GNU sed and MacOS sed.

Previous use of sed may have only worked on MacOS.

See #52

update_academic.sh

@@ -31,7 +31,7 @@ function do_update () {
     version=$(sed -n 's/^min_version = //p' themes/academic/theme.toml | tr -d '"')
     version="${version}.0"
     echo "Set Netlify Hugo version to v${version}"
-    sed -i '' -e "s/HUGO_VERSION = .*/HUGO_VERSION = \"$version\"/g" ./netlify.toml
+    sed -i.bak -e "s/HUGO_VERSION = .*/HUGO_VERSION = \"$version\"/g" ./netlify.toml && rm -f ./netlify.toml.bak
   fi
 
   echo