fix: correct undefined variable reference in error handler

Fix ReferenceError caused by referencing undefined variable 'e' instead
of the correctly scoped 'error' variable in the catch block.

Before: catch (error) { ... e.message }
After:  catch (error) { ... error.message }

This bug prevented proper error responses when signature generation failed,
causing the error handler itself to throw an exception.

Hacktoberfest contribution

Author: Mrunal Chawda

docs/pages/api/sign-cloudinary-params.js

@@ -13,7 +13,7 @@ export default async function handler(req, res) {
     });
   } catch (error) {
     res.status(500).json({
-      error: e.message,
+      error: error.message,
     });
   }
 }