plugins/t/check_dns.t: Fix Perl warning

Perl said: "Unrecognized escape \d passed through at ./t/check_dns.t
line 57."

plugins/t/check_dns.t

@@ -54,7 +54,7 @@ cmp_ok( $res->return_code, '==', 2, "Critical threshold passed");
 
 $res = NPTest->testCmd("./check_dns -H $hostname_valid -t 5 -w 0 -c 5");
 cmp_ok( $res->return_code, '==', 1, "Warning threshold passed");
-like( $res->output, "/\|time=[\d\.]+s;0.0*;5\.0*;0\.0*/", "Output performance data OK" );
+like( $res->output, '/\|time=[\d\.]+s;0.0*;5\.0*;0\.0*/', "Output performance data OK" );
 
 $res = NPTest->testCmd("./check_dns -H $hostname_invalid -t 1");
 cmp_ok( $res->return_code, '==', 2, "Invalid $hostname_invalid");