docs: Fix typo (#2627)

Author: KATO-Hiro

docs/dev-notes/2025-09-23/contest-task-pair-mapping/plan.md

@@ -100,7 +100,7 @@ export async function createContestTaskPair(
       },
     });
   } catch (error) {
-    if (error instanceof PrismaClientKnownRequestError && error.code === 'P2002') {
+    if (error instanceof Prisma.PrismaClientKnownRequestError && error.code === 'P2002') {
       const errorMessage = `ContestTaskPair already exists: contestId=${contestId}, taskId=${taskId}`;
       console.error(errorMessage);
       throw new Error(errorMessage);
@@ -132,7 +132,7 @@ export async function updateContestTaskPair(
       },
     });
   } catch (error) {
-    if (error instanceof PrismaClientKnownRequestError && error.code === 'P2025') {
+    if (error instanceof Prisma.PrismaClientKnownRequestError && error.code === 'P2025') {
       const errorMessage = `Not found ContestTaskPair: contestId=${contestId}, taskId=${taskId}`;
       console.error(errorMessage);
       throw new Error(errorMessage);