[lex.separate] Remove last reference to instantation units

The term instantiation unit was removed by the reflection paper, P2996.

Author: AlisdairM

source/lex.tex

@@ -43,10 +43,8 @@
 
 \pnum
 \begin{note}
-Previously translated translation units and instantiation
-units can be preserved individually or in libraries. The separate
-translation units of a program communicate\iref{basic.link} by (for
-example)
+Previously translated translation units can be preserved individually or in libraries.
+The separate translation units of a program communicate\iref{basic.link} by (for example)
 calls to functions whose identifiers have external or module linkage,
 manipulation of objects whose identifiers have external or module linkage, or
 manipulation of data files. Translation units can be separately