Fixes #23376 Broken URL for Centos yum repo for docker changed to now have a hardcoded centos version 7

Signed-off-by: Shoubhik Bose <sbose78@gmail.com>
(cherry picked from commit d3fad80cc7823eddb823dd53424a28ee81043f18)

docs/installation/linux/centos.md

@@ -57,7 +57,7 @@ package manager.
         $ sudo tee /etc/yum.repos.d/docker.repo <<-'EOF'
         [dockerrepo]
         name=Docker Repository
-        baseurl=https://yum.dockerproject.org/repo/main/centos/$releasever/
+        baseurl=https://yum.dockerproject.org/repo/main/centos/7/
         enabled=1
         gpgcheck=1
         gpgkey=https://yum.dockerproject.org/gpg