Fix spelling mistakes.

Same patch submitted independently by David Rowley and Peter Geoghegan.

src/backend/executor/nodeGather.c

@@ -138,7 +138,7 @@ ExecGather(GatherState *node)
 	/*
 	 * Initialize the parallel context and workers on first execution. We do
 	 * this on first execution rather than during node initialization, as it
-	 * needs to allocate large dynamic segement, so it is better to do if it
+	 * needs to allocate large dynamic segment, so it is better to do if it
 	 * is really needed.
 	 */
 	if (!node->initialized)

src/bin/pg_upgrade/controldata.c

@@ -539,7 +539,7 @@ check_control_data(ControlData *oldctrl,
 		pg_fatal("old and new pg_controldata block sizes are invalid or do not match\n");
 
 	if (oldctrl->largesz == 0 || oldctrl->largesz != newctrl->largesz)
-		pg_fatal("old and new pg_controldata maximum relation segement sizes are invalid or do not match\n");
+		pg_fatal("old and new pg_controldata maximum relation segment sizes are invalid or do not match\n");
 
 	if (oldctrl->walsz == 0 || oldctrl->walsz != newctrl->walsz)
 		pg_fatal("old and new pg_controldata WAL block sizes are invalid or do not match\n");