[SYN-4452] Update private-locations page for watchtower update

[purvitm-splunk]

Requirements

• Content follows Splunk guidelines for style and formatting.
• You are contributing original content.

Describe the change

Updated the private-location document to Automatically Update Private Location Runners - Using Watchtower

[gschatz-splunk]

hi! thank you so much for your enthusiasm, this is great. As I mentioned in slack I am closing this as it is a duplicate MR and I have one going with some other updates in line with our style guide. Thanks so much for your help and I look forward to more collaboration in the future :)