Update manual upgrade URL (#166)

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Following:

1. [ ] Are internal links to wiki documents using [relative file
links](https://docusaurus.io/docs/markdown-features/links)?
2. [ ] Are all new documentation files lowercase, with dash separated
names (ex. unraid-os.mdx)?
3. [ ] Are all assets (images, etc), located in an assets/ subfolder
next to the .md/mdx files?
4. [ ] Have you checked to ensure there aren't other open [Pull
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5. [ ] Is the build succeeding?

docs/legacy/Articles/upgrading-to-unraid-6.md

@@ -36,7 +36,7 @@ In this case, manually install the update as follows:
 
 1. Navigate to Plugins/Install Plugin tab
 2. Select/copy this text
-   [`https://raw.githubusercontent.com/limetech/unRAIDServer/master/unRAIDServer.plg`](https://raw.githubusercontent.com/limetech/unRAIDServer/master/unRAIDServer.plg)
+   `https://stable.dl.unraid.net/unRAIDServer.plg`
 3. Paste into URL field and click Install
 
 ### My array / docker apps are taking a really long time to start after the upgrade

docs/unraid-os/manual/upgrade-instructions.md

@@ -55,7 +55,7 @@ In this case, manually install the update as follows:
 
 1. Navigate to Plugins/Install Plugin tab
 2. Select/copy this text
-   https://s3.amazonaws.com/dnld.lime-technology.com/stable/unRAIDServer.plg
+   `https://stable.dl.unraid.net/unRAIDServer.plg`
 3. Paste into URL field and click Install
 
 #### My array/docker apps are taking a really long time to start after the upgrade