um: Fix comment typo

The double `in' is duplicated in line 172, remove one.
Signed-off-by: Jason Wang <wangborong@cdjrlc.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/kernel/physmem.c

@@ -169,7 +169,7 @@ __uml_setup("iomem=", parse_iomem,
 );
 
 /*
- * This list is constructed in parse_iomem and addresses filled in in
+ * This list is constructed in parse_iomem and addresses filled in
  * setup_iomem, both of which run during early boot.  Afterwards, it's
  * unchanged.
  */