Update installation.rst (#3074)

* Update installation.rst

##### SUMMARY
centos8 prefers the usage of dnf instead of yum
ubuntu prefers the use of apt above apt-get for cli 


##### ISSUE TYPE
- Docs Pull Request

+label: docsite_pr

* Update installation.rst

* Update installation.rst

docs/installation.rst

@@ -21,15 +21,15 @@ CentOS 8
 
 .. code-block:: bash
 
-    $ sudo yum install -y gcc python3-pip python3-devel openssl-devel python3-libselinux
+    $ sudo dnf install -y gcc python3-pip python3-devel openssl-devel python3-libselinux
 
 Ubuntu 16.x
 -----------
 
 .. code-block:: bash
 
-    $ sudo apt-get update
-    $ sudo apt-get install -y python3-pip libssl-dev
+    $ sudo apt update
+    $ sudo apt install -y python3-pip libssl-dev
 
 Pip
 ===
@@ -176,14 +176,14 @@ CentOS 8
 
 .. code-block:: bash
 
-    $ sudo yum install -y libffi-devel git
+    $ sudo dnf install -y libffi-devel git
 
 Ubuntu 16.x
 ^^^^^^^^^^^
 
 .. code-block:: bash
 
-    $ sudo apt-get install -y libffi-dev git
+    $ sudo apt install -y libffi-dev git
 
 Install
 -------