Adjust operation names of pg_aios to match the documentation

pg_aios used the terms "read" and "write" for vectored I/O read and
write operations, respectively.  The documentation refers to them as
"readv" and "writev", and the code uses internally the terms
PGAIO_OP_READV and PGAIO_OP_WRITEV for them, as of "vectored".

This commit adjusts these operation names to match with the code and the
documentation.

Oversight in 8e293e689b.

Author: Atsushi Torikoshi <torikoshia@oss.nttdata.com>
Discussion: https://postgr.es/m/6df1e949d1d759ad2767c18e5845963e@oss.nttdata.com

src/backend/storage/aio/aio_io.c

@@ -181,9 +181,9 @@ pgaio_io_get_op_name(PgAioHandle *ioh)
 		case PGAIO_OP_INVALID:
 			return "invalid";
 		case PGAIO_OP_READV:
-			return "read";
+			return "readv";
 		case PGAIO_OP_WRITEV:
-			return "write";
+			return "writev";
 	}
 
 	return NULL;				/* silence compiler */