Fix typo.

src/backend/executor/execTuples.c

@@ -140,7 +140,7 @@ tts_virtual_getsysattr(TupleTableSlot *slot, int attnum, bool *isnull)
  * To materialize a virtual slot all the datums that aren't passed by value
  * have to be copied into the slot's memory context.  To do so, compute the
  * required size, and allocate enough memory to store all attributes.  That's
- * good for cache hit ratio, but more imporantly requires only memory
+ * good for cache hit ratio, but more importantly requires only memory
  * allocation/deallocation.
  */
 static void