[stmt.if] Fix spelling mistake in 'if constexpr' wording. (#1653)

Author: lewissbaker

source/statements.tex

@@ -255,7 +255,7 @@
 form is called a \defn{constexpr if} statement. If the value of the
 converted condition is \tcode{false}, the first substatement is a
 \defn{discarded statement}, otherwise the second substatement, if
-present, is a discarded statement. During the instantation of an
+present, is a discarded statement. During the instantiation of an
 enclosing templated entity (Clause~\ref{temp}), if the condition is
 not value-dependent after its instantiation, the discarded substatement
 (if any) is not instantiated.