ShellPkg: Fix spelling mistake for occurred

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2361



Cc: Ray Ni <ray.ni@intel.com>
Cc: Zhichao Gao <zhichao.gao@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Guomin Jiang <guomin.jiang@intel.com>
Reviewed-by: Zhichao Gao <zhichao.gao@intel.com>