Use correct spelling of statistics kind

A couple error messages and comments used 'statistic kind', not the
correct 'statistics kind'. Fix and backpatch all the way back to 10,
where extended statistics were introduced.

Backpatch-through: 10

doc/src/sgml/catalogs.sgml

@@ -6575,7 +6575,7 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
       <entry><type>char[]</type></entry>
       <entry></entry>
       <entry>
-        An array containing codes for the enabled statistic kinds;
+        An array containing codes for the enabled statistics kinds;
         valid values are:
         <literal>d</literal> for n-distinct statistics,
         <literal>f</literal> for functional dependency statistics

src/backend/statistics/dependencies.c

@@ -642,7 +642,7 @@ statext_dependencies_load(Oid mvoid)
 						   Anum_pg_statistic_ext_stxdependencies, &isnull);
 	if (isnull)
 		elog(ERROR,
-			 "requested statistic kind \"%c\" is not yet built for statistics object %u",
+			 "requested statistics kind \"%c\" is not yet built for statistics object %u",
 			 STATS_EXT_DEPENDENCIES, mvoid);
 
 	result = statext_dependencies_deserialize(DatumGetByteaPP(deps));

src/backend/statistics/extended_stats.c

@@ -44,7 +44,7 @@ typedef struct StatExtEntry
 	char	   *schema;			/* statistics object's schema */
 	char	   *name;			/* statistics object's name */
 	Bitmapset  *columns;		/* attribute numbers covered by the object */
-	List	   *types;			/* 'char' list of enabled statistic kinds */
+	List	   *types;			/* 'char' list of enabled statistics kinds */
 } StatExtEntry;
 
 

src/backend/statistics/mvdistinct.c

@@ -139,7 +139,7 @@ statext_ndistinct_load(Oid mvoid)
 							Anum_pg_statistic_ext_stxndistinct, &isnull);
 	if (isnull)
 		elog(ERROR,
-			 "requested statistic kind \"%c\" is not yet built for statistics object %u",
+			 "requested statistics kind \"%c\" is not yet built for statistics object %u",
 			 STATS_EXT_NDISTINCT, mvoid);
 
 	result = statext_ndistinct_deserialize(DatumGetByteaPP(ndist));

src/include/nodes/relation.h

@@ -857,7 +857,7 @@ typedef struct StatisticExtInfo
 
 	Oid			statOid;		/* OID of the statistics row */
 	RelOptInfo *rel;			/* back-link to statistic's table */
-	char		kind;			/* statistic kind of this entry */
+	char		kind;			/* statistics kind of this entry */
 	Bitmapset  *keys;			/* attnums of the columns covered */
 } StatisticExtInfo;