Fix a broken link from AIP 132 to AIP 156 (#1045)

Author: x746e

aip/general/0132.md

@@ -198,7 +198,7 @@ NOT_FOUND errors][permission-denied].
 [aip-203]: ./0203.md
 [errors]: ./0193.md
 [permission-denied]: ./0193.md#permission-denied
-[singleton]: ./156.md
+[singleton]: ./0156.md
 [soft delete]: ./0135.md#soft-delete
 
 ## Changelog