[lex.phases] Fix typo of "instantiation"

The word should be "instantiation" but not "instantation".

Author: frederick-vs-ja

source/lex.tex

@@ -222,7 +222,7 @@
 Other requirements in this document can further constrain
 the context from which an instantiation can be performed.
 For example, a constexpr function template specialization
-might have a point of instantation at the end of a translation unit,
+might have a point of instantiation at the end of a translation unit,
 but its use in certain constant expressions could require
 that it be instantiated at an earlier point\iref{temp.inst}.
 \end{note}