Update the API doc generation script to use the latest pdoc3

Author: seratch

scripts/generate_api_docs.sh

@@ -4,7 +4,7 @@
 script_dir=`dirname $0`
 cd ${script_dir}/..
 
-pip install pdoc3
+pip install -U pdoc3
 rm -rf docs/api-docs
 pdoc slack_sdk --html -o docs/api-docs
 open docs/api-docs/slack_sdk/index.html