Fix name of argument to pg_stat_file.

It's called "missing_ok" in the docs and in the C code.

I refrained from doing a catversion bump for this, because the name of an
input argument is just documentation, it has no effect on any callers.

Michael Paquier

src/include/catalog/pg_proc.h

@@ -3183,7 +3183,7 @@ DESCR("rotate log file");
 
 DATA(insert OID = 2623 ( pg_stat_file		PGNSP PGUID 12 1 0 0 0 f f f f t f v 1 0 2249 "25" "{25,20,1184,1184,1184,1184,16}" "{i,o,o,o,o,o,o}" "{filename,size,access,modification,change,creation,isdir}" _null_ _null_ pg_stat_file_1arg _null_ _null_ _null_ ));
 DESCR("get information about file");
-DATA(insert OID = 3307 ( pg_stat_file		PGNSP PGUID 12 1 0 0 0 f f f f t f v 2 0 2249 "25 16" "{25,16,20,1184,1184,1184,1184,16}" "{i,i,o,o,o,o,o,o}" "{filename,if_not_exists,size,access,modification,change,creation,isdir}" _null_ _null_ pg_stat_file _null_ _null_ _null_ ));
+DATA(insert OID = 3307 ( pg_stat_file		PGNSP PGUID 12 1 0 0 0 f f f f t f v 2 0 2249 "25 16" "{25,16,20,1184,1184,1184,1184,16}" "{i,i,o,o,o,o,o,o}" "{filename,missing_ok,size,access,modification,change,creation,isdir}" _null_ _null_ pg_stat_file _null_ _null_ _null_ ));
 DESCR("get information about file");
 DATA(insert OID = 2624 ( pg_read_file		PGNSP PGUID 12 1 0 0 0 f f f f t f v 3 0 25 "25 20 20" _null_ _null_ _null_ _null_ _null_ pg_read_file_off_len _null_ _null_ _null_ ));
 DESCR("read text from a file");