Fix typos "are are".

src/include/nodes/execnodes.h

@@ -982,7 +982,7 @@ typedef struct PlanState
 } PlanState;
 
 /* ----------------
- *	these are are defined to avoid confusion problems with "left"
+ *	these are defined to avoid confusion problems with "left"
  *	and "right" and "inner" and "outer".  The convention is that
  *	the "left" plan is the "outer" plan and the "right" plan is
  *	the inner plan, but these make the code more readable.

src/include/nodes/plannodes.h

@@ -127,7 +127,7 @@ typedef struct Plan
 } Plan;
 
 /* ----------------
- *	these are are defined to avoid confusion problems with "left"
+ *	these are defined to avoid confusion problems with "left"
  *	and "right" and "inner" and "outer".  The convention is that
  *	the "left" plan is the "outer" plan and the "right" plan is
  *	the inner plan, but these make the code more readable.