doc: Fix 'RTL SSA Access Lists' description

In the 'RTL SSA Access Lists' subsection of
GCC's documentation, an example of code with
an 'if'...'else' structure is given. The
given list of the full list of accesses is
said to include "use of the ebb4's R phi
definition of R by B". That cannot be true
because B is in the first substatement of the
'if' statement, whereas ebb4 is the second
substatement. First and second substatements
are mutually exclusive: there is no path to
B that goes through the degenerate phi node
of ebb4.

It looks as though D was intended, not B.

gcc/ChangeLog:

	* doc/rtl.texi: Fix example of the SSA form.

gcc/doc/rtl.texi

@@ -4843,7 +4843,7 @@ definition of R by ebb4's phi instruction, with the input coming from A
 
 @itemize @bullet
 @item
-use of the ebb4's R phi definition of R by B
+use of ebb4's phi definition of R by D
 @end itemize
 @end itemize