Typo: ibatomic_ops -> libatomic_ops

Change-Id: Iaa4ff5cd30d3163e7a44f162bbe296c59a7111cb
Reviewed-on: https://gerrit.libreoffice.org/69514
Tested-by: Jenkins
Reviewed-by: Andras Timar <andras.timar@collabora.com>

external/libatomic_ops/README

 From [https://github.com/ivmai/libatomic_ops/].
 
 Note that some files (linked into libatomic_ops_gpl) are GPL'd, the portion
-needed for Firebird (ibatomic_ops) is however MIT licensed. 
+needed for Firebird (libatomic_ops) is however MIT licensed.