[Ada] Update documentation of Sinfo.Package_Instantiation field

This was forgotten when renaming Get_Package_Instantiation_Node to
Get_Unit_Instantiation_Node.

2018-05-30  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* einfo.ads (Package_Instantiation): Update comment after a routine
	that uses this field has been renamed.

From-SVN: r260950

gcc/ada/ChangeLog

+2018-05-30  Piotr Trojanek  <trojanek@adacore.com>
+
+	* einfo.ads (Package_Instantiation): Update comment after a routine
+	that uses this field has been renamed.
+
 2018-05-30  Ed Schonberg  <schonberg@adacore.com>
 
 	* exp_ch11.adb (Replace_Raise_By_Goto): The transfomation is legal even

gcc/ada/einfo.ads

@@ -3838,7 +3838,7 @@ package Einfo is
 --       been rewritten to an N_Package_Specification, the instantiation
 --       node is available from the Original_Node field of the package spec
 --       node. This is currently not guaranteed to be set in all cases, but
---       when set, the field is used in Get_Package_Instantiation_Node as
+--       when set, the field is used in Get_Unit_Instantiation_Node as
 --       one of the means of obtaining the instantiation node. Eventually
 --       it should be set in all cases, including package entities associated
 --       with formal packages. ???