Update main developer guide (#4378)

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#### What type of PR is this?

docs

#### What this PR does / why we need it:

Fixes the `docs/developer/ui.md` reference.

#### Which issue(s) this PR fixes:

#### Special notes for your reviewer:

Signed-off-by: Emmanuel Ferdman <[email protected]>

Author: emmanuel-ferdman

internal/lookoutui/README.md

@@ -54,7 +54,7 @@ real-time. This serves the web app on
 [http://localhost:3000](http://localhost:3000). It proxies API requests to the
 target defined by your `PROXY_TARGET` environment variable, or otherwise your
 locally-running instance of the Lookout server at `http://localhost:10000`
-(please see [the main developer docs](../../../docs/developer/ui.md) for details
+(please see [the main developer docs](../../docs/developer/ui.md) for details
 of how to set this up).
 
 ```bash