fix sed for MacOS (#36)

Close #4

Right now on MacOS, sed will produce backup files, because on Mac, If do something like below using bash script

```
$SED_INPLACE '1s/---/---\nisOutdated: true/' $file
```

It will become the following on executed, and a backup file will be produced:

```
sed -i ''\'''\''' '1s/---/---\nisOutdated: true/' ./docs/content/doc/help/faq.zh-cn.md
```

The way to fix this in this PR is to change the `SED_INPLACE` to a function. [Reference](aa19c2d125/generate-cpp.sh (L4)) (Tested on my
Mac and backup files with suffix '' will not be produced after changing to function)

Reviewed-on: https://gitea.com/gitea/gitea-docusaurus/pulls/36
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: HesterG <hestergong@gmail.com>
Co-committed-by: HesterG <hestergong@gmail.com>

check_outdated.sh

@@ -9,11 +9,14 @@
 
 set -xe
 
-if sed --version 2>/dev/null | grep -q GNU; then
-    SED_INPLACE="sed -i"
-else
-    SED_INPLACE="sed -i ''"
-fi
+SED_INPLACE() {
+  if sed --version 2>/dev/null | grep -q GNU; then
+    sed -i "$@"
+  else
+    sed -i '' "$@"
+  fi
+}
+
 version="$1"
 locale="$2"
 cur_path=`pwd`
@@ -31,7 +34,7 @@ for file in `find ./docs/content/doc -name "*.${locale}.md"`; do
     fi
     if [[ "$latest_commit_time_en" -gt "$latest_commit_time_locale" ]]; then
         echo "file: $file, lastest commit timestamp: $latest_commit_time_en (en ver), $latest_commit_time_locale ($locale ver)"
-        $SED_INPLACE '1s/---/---\nisOutdated: true/' $file
+        SED_INPLACE '1s/---/---\nisOutdated: true/' $file
     fi
 done