docs: fix broken /references/custom-fields link to /guides/custom-fields

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: torimwhaley

guides/ai-agents/using-ai-agents.mdx

@@ -75,7 +75,7 @@ When you ask a comparison question, the agent picks a base metric, a time dimens
 - *"Year-over-year orders by week"*
 - *"Show revenue this quarter vs the previous quarter, and vs the same quarter last year"*
 
-You can stack multiple comparisons on the same metric (for example, MoM and YoY together) and you can use a [custom metric](/references/custom-fields) as the base.
+You can stack multiple comparisons on the same metric (for example, MoM and YoY together) and you can use a [custom metric](/guides/custom-fields) as the base.
 
 <Tip>
   Period-over-period is for comparing whole prior periods (last month, last quarter, last year). For period-to-date questions like MTD or YTD, ask the agent for a filtered aggregation instead — for example, *"revenue so far this month vs the same point last month"*.