doc: update 'asa.rst' for 2.6 release
Update security vulnerability fix in 2.6 release.
Security Advisory
#################
+Addressed in ACRN v2.6
+************************
+
+We recommend that all developers upgrade to this v2.6 release (or later), which
+addresses the following security issue discovered in previous releases:
+
+-----
+
+- Memory leakage vulnerability in ``devicemodel/hw/pci/xhci.c``
+ De-initializing of emulated USB devices results in a memory leakage issue
+ as some resources allocated for transfer are not properly released.
+
+ **Affected Release:** v2.5 and earlier.
+
+
Addressed in ACRN v2.5
************************
