docs(eslint-plugin): [no-unsafe-member-access] correct example idx fr…

…om const to let (#7833)

Update no-unsafe-member-access.md

Update variable declaration from const to let

packages/eslint-plugin/docs/rules/no-unsafe-member-access.md

@@ -54,7 +54,7 @@ properlyTyped.prop[key];
 
 const arr = [1, 2, 3];
 arr[1];
-const idx = 1;
+let idx = 1;
 arr[idx];
 arr[idx++];
 ```