[lex.separate] Remove last reference to instantation units

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

Author: AlisdairM

source/lex.tex

@@ -43,8 +43,8 @@
 
 \pnum
 \begin{note}
-Previously translated translation units and instantiation
-units can be preserved individually or in libraries. The separate
+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,