sandbox: Correct a typo in mapmem

This should say 'cast', not 'case', so fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/mapmem.h b/include/mapmem.h
index f496c96..8ec03d5 100644
--- a/include/mapmem.h
+++ b/include/mapmem.h
@@ -33,7 +33,7 @@
  * nomap_sysmem() - pass through an address unchanged
  *
  * This is used to indicate an address which should NOT be mapped, e.g. in
- * SMBIOS tables. Using this function instead of a case shows that the sandbox
+ * SMBIOS tables. Using this function instead of a cast shows that the sandbox
  * conversion has been done
  */
 static inline void *nomap_sysmem(phys_addr_t paddr, unsigned long len)