docs: fix variable shadowing in management API overview example

The list example declared `const runs = await runs.list(...)`, which
shadowed the imported `runs` and threw at runtime. Rename the local to
`completedRuns`.

Author: D-K-P

docs/management/overview.mdx

@@ -37,7 +37,7 @@ configure({
 });
 
 async function main() {
-  const runs = await runs.list({
+  const completedRuns = await runs.list({
     limit: 10,
     status: ["COMPLETED"],
   });