MINOR: stats: fix minor typo fix in stats_dump_errors_to_buffer()

Remove the space before the colon to match the format used in the frontend.
diff --git a/src/dumpstats.c b/src/dumpstats.c
index 3855e09..ebf66ec 100644
--- a/src/dumpstats.c
+++ b/src/dumpstats.c
@@ -6045,7 +6045,7 @@
 				break;
 			case 1:
 				chunk_appendf(&trash,
-					     " backend %s (#%d) : invalid response\n"
+					     " backend %s (#%d): invalid response\n"
 					     "  frontend %s (#%d)",
 					     appctx->ctx.errors.px->id, appctx->ctx.errors.px->uuid,
 					     es->oe->id, es->oe->uuid);