Fix description of I/O timing info for shared buffers in EXPLAIN (BUFFERS)

This fixes an error introduced by efb0ef909f, that changed the
description of this field to "shared/local" while these I/O timings
relate to shared buffers.  This information is available when
track_io_timing is enabled.  Note that HEAD has added new counters for
local buffers in 295c36c0c1, so there is no need to touch it.  The
description is updated to "shared" to be compatible with HEAD.

Per discussion with Nazir Bilal Yavuz and Hubert Depesz Lubaczewski,
whose EXPLAIN analyzer tool was not actually able to parse the previous
term because of the slash character.

Discussion: https://postgr.es/m/ZTCTiUqm_H3iBihl@paquier.xyz
Backpatch-through: 15

src/backend/commands/explain.c

@@ -3632,7 +3632,7 @@ show_buffer_usage(ExplainState *es, const BufferUsage *usage, bool planning)
 
 			if (has_timing)
 			{
-				appendStringInfoString(es->str, " shared/local");
+				appendStringInfoString(es->str, " shared");
 				if (!INSTR_TIME_IS_ZERO(usage->blk_read_time))
 					appendStringInfo(es->str, " read=%0.3f",
 									 INSTR_TIME_GET_MILLISEC(usage->blk_read_time));