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>

ShellPkg/Library/UefiShellDriver1CommandsLib/Drivers.c

@@ -22,7 +22,7 @@ STATIC CONST SHELL_PARAM_ITEM ParamList[] = {
 
   @param[in] TheHandle      The handle to get the device path for.
 
-  @retval NULL    An error occured.
+  @retval NULL    An error occurred.
   @return         A pointer to the driver path as a string.  The callee must
                   free this memory.
 **/