SourceLevelDebugPkg/Readme.txt: Update UDK Debugger Tool & manual URL
REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1289
The commit updates the link to get access to the Intel(R) UDK Debugger
Tool and its detailed user manual.
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>