Fix 0017870: XSS in adm_config_report.php

This is the real correct fix for this issue (i.e. using string_attribute()
to escape the variable), which was supposed to have been fixed in commit
1a49a780a7881b6eb7a2384a432db072a5c6db79.

Unfortunately, for some reason I somehow ended up redoing the same
mistake of using string_display_line() again instead (see original fix
b509ab380f91e84d9683dbfdb02206b88a9b86fb, reverted in
b02557d88a2094330249dc06c2990184d2696372).

It is worth mentioning that string_display_line() does protect against
the XSS attack vector, provided that the relevant MantisBT Formatting
plugin configuration (text processing) is set to ON.

Thanks to Patrice Morineau for pointing this out.