Fix minor bug in dap-support.exp

Commit faee137249c8 refactored dap-support.exp, but accidentally left
a reference to a variable that was renamed.  I noticed this when an
earlier version of this series provoked one of the errors in this
code.

gdb/testsuite/lib/dap-support.exp

diff --git a/gdb/testsuite/lib/dap-support.exp b/gdb/testsuite/lib/dap-support.exp
index 2d962e6..5c078ca 100644
--- a/gdb/testsuite/lib/dap-support.exp
+++ b/gdb/testsuite/lib/dap-support.exp
@@ -231,9 +231,9 @@
 	set d [_dap_read_json]
 	if {[dict get $d type] == "response"} {
 	    if {[dict get $d request_seq] != $num} {
-		error "saw wrong request_seq in $obj"
+		error "saw wrong request_seq in $d"
 	    } elseif {[dict get $d command] != $cmd} {
-		error "saw wrong command in $obj"
+		error "saw wrong command in $d"
 	    } else {
 		return [list $d $result]
 	    }