Fix tests to use the right field name

It doesn't matter for what this test used to test (that nothing changes
if the 'pagers' array exists), but it will be relevant once we further
migrate the 'pagers' array from there.

pkg/config/app_config_test.go

@@ -618,13 +618,13 @@ func TestPagerMigration(t *testing.T) {
   paging:
     pager: delta --dark --paging=never
   pagers:
-    - diff: diff-so-fancy
+    - pager: diff-so-fancy
 `,
 			expected: `git:
   paging:
     pager: delta --dark --paging=never
   pagers:
-    - diff: diff-so-fancy
+    - pager: diff-so-fancy
 `,
 			expectedDidChange: false,
 			expectedChanges:   []string{},