Adjust some error hints

The wording of two error hints is tweaked in this commit:
- Import of extended statistics, where the value of an array element is
not a NULL or a string.
- Online data checksum switch, where a period was missing.

Author:  Baji Shaik <baji.pgdev@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CA+fm-RMrKbyky_+vi5SDdAVnFVjWh7zW3GoDAVnrp5OpDnW6tw@mail.gmail.com

src/backend/access/transam/xlog.c

@@ -6612,7 +6612,7 @@ StartupXLOG(void)
 
 		ereport(WARNING,
 				errmsg("enabling data checksums was interrupted"),
-				errhint("Data checksum processing must be manually restarted for checksums to be enabled"));
+				errhint("Data checksum processing must be manually restarted for checksums to be enabled."));
 	}
 
 	/*

src/backend/statistics/extended_stats_funcs.c

@@ -1160,7 +1160,7 @@ import_pg_statistic(Relation pgsd, JsonbContainer *cont,
 				ereport(WARNING,
 						errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 						errmsg("could not parse \"%s\": invalid element in expression %d", argname, exprnum),
-						errhint("Value of element \"%s\" must be type a null or a string.", s));
+						errhint("Value of element \"%s\" must be a null or a string.", s));
 				goto pg_statistic_error;
 		}
 	}

src/test/regress/expected/stats_import.out

@@ -2493,7 +2493,7 @@ SELECT pg_catalog.pg_restore_extended_stats(
   'exprs', '[ { "null_frac": 1 },
               { "null_frac": "0.25" } ]'::jsonb);
 WARNING:  could not parse "exprs": invalid element in expression -1
-HINT:  Value of element "null_frac" must be type a null or a string.
+HINT:  Value of element "null_frac" must be a null or a string.
  pg_restore_extended_stats 
 ---------------------------
  f