Fix setup-centos8 script to use pip3.9 (facebookincubator#9536)

Summary:
Use pip3.9 to install packages since python39-pip is installed.

Pull Request resolved: facebookincubator#9536

Reviewed By: Yuhta

Differential Revision: D56332732

Pulled By: xiaoxmeng

fbshipit-source-id: b78d034e81e6f3f8b8569a1e3cf5334f40de81c2

scripts/setup-centos8.sh

@@ -61,7 +61,7 @@ function install_velox_deps_from_dnf {
     libdwarf-devel curl-devel libicu-devel bison flex libsodium-devel
 
   # install sphinx for doc gen
-  pip3 install sphinx sphinx-tabs breathe sphinx_rtd_theme
+  pip3.9 install sphinx sphinx-tabs breathe sphinx_rtd_theme
 }
 
 function install_conda {