Fix typo in comment.

Author: Haiying Tang <tanghy.fnst@cn.fujitsu.com>
Discussion: https://postgr.es/m/48a0928ac94b497d9c40acf1de394c15@G08CNEXMBPEKD05.g08.fujitsu.local

src/include/optimizer/cost.h

@@ -20,7 +20,7 @@
 
 /* defaults for costsize.c's Cost parameters */
 /* NB: cost-estimation code should use the variables, not these constants! */
-/* If you change these, update backend/utils/misc/postgresql.sample.conf */
+/* If you change these, update backend/utils/misc/postgresql.conf.sample */
 #define DEFAULT_SEQ_PAGE_COST  1.0
 #define DEFAULT_RANDOM_PAGE_COST  4.0
 #define DEFAULT_CPU_TUPLE_COST	0.01