Fix the comment describing b_list (GH-28925)

b_list points to the previously allocated block, not to the next one.

Author: sl4mmy

Python/flowgraph.c

@@ -41,9 +41,9 @@ typedef struct _PyCfgInstruction {
 
 typedef struct _PyCfgBasicblock {
     /* Each basicblock in a compilation unit is linked via b_list in the
-       reverse order that the block are allocated.  b_list points to the next
-       block in this list, not to be confused with b_next, which is next by
-       control flow. */
+       reverse order that blocks are allocated.  b_list points to the
+       previously allocated block, not to be confused with b_next, which is
+       next by control flow. */
     struct _PyCfgBasicblock *b_list;
     /* The label of this block if it is a jump target, -1 otherwise */
     _PyJumpTargetLabel b_label;