md.texi: fix typo in save_stack_* node.

From-SVN: r20869

gcc/md.texi

@@ -2414,7 +2414,7 @@ such required data.
 When saving the stack pointer, operand 0 is the save area and operand 1
 is the stack pointer.  The mode used to allocate the save area defaults
 to @code{Pmode} but you can override that choice by defining the
-@code{STACK_SAVEAREA_MODE} macro (@pxref{Storage_Layout}).  You must
+@code{STACK_SAVEAREA_MODE} macro (@pxref{Storage Layout}).  You must
 specify an integral mode, or @code{VOIDmode} if no save area is needed
 for a particular type of save (either because no save is needed or
 because a machine-specific save area can be used).  Operand 0 is the