[TYPO] Duplicate "NGINX Plus" in Docker install documentation

[tusharpatil03]

File:
content/nginx/admin-guide/installing-nginx/installing-nginx-docker.md

Section:
Use official NGINX Plus Docker images

Issue:
There is a typographical error. The sentence reads:

Since NGINX Plus NGINX Plus [Release 31]({{< ref "nginx/releases.md#r31" >}}) you can get an NGINX Plus image from the official NGINX Plus Docker registry and upload it to your private registry.

Correction:
It should be:

Since NGINX Plus [Release 31]({{< ref "nginx/releases.md#r31" >}}), you can get an NGINX Plus image from the official NGINX Plus Docker registry and upload it to your private registry.