[DOCS] CentOS 7 incorrect installation location

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.6/static/tutorial-sql-intro.html
Description:

The make command in the tutorial throws an error concerning the pgxs path.
"pg_config --pgxs" returns "/usr/lib/pgxs/...", when the actual path is
"/usr/pgsql-9.6/lib/pgxs/...". Similarly, the expected location of the
header files is "/usr/include/...", when they are actually installed to
"/usr/pgsql-9.6/include/...". This causes the command to fail and so the
tutorial cannot continue until the issues are addressed.

To replicate:
1. Install postgres repo
"yum install
https://download.postgresql.org/pub/repos/yum/9.6/redhat/rhel-7-x86_64/pgdg-centos96-9.6-3.noarch.rpm"
2. Install postgres and postgres-server
"yum install postgresql96 postgresql96-devel postgresql96-server"
3. Start postgresql server using initdb/systemctl commands
4. Download postgresql-9.6 source from
https://ftp.postgresql.org/pub/source/v9.6.1/postgresql-9.6.1.tar.gz
5. Expand source files
6. Move to tutorial directory
"cd ..../src/tutorial"
7. Run make