ShellPkg/cd: Fix "cd" to support "fs0:dir" (no slash after ':')

ShellPkg/cd: Fix "cd" to support "fs0:dir" (no slash after ':')

When "fs0:dir"(drive letter without slash) is used as destination
of "cd", "cd" tries to change to "dir" in root directory of "fs0:".
It's incorrect. The correct behavior is to change to "dir" in
current directory of "fs0:"

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Signed-off-by: Chen A Chen <chen.a.chen@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>