[temp.constr.normal] Remove duplicate "the" (#7135)

Author: Andreas-Krug

source/templates.tex

@@ -1948,7 +1948,7 @@
 that is either \tcode{\&\&} or \tcode{||},
 the normal form of an expression
 \tcode{( E1 \grammarterm{fold-operator} ... \grammarterm{fold-operator} E2 )}
-is the the normal form of
+is the normal form of
 \begin{itemize}
 \item
 \tcode{( E1 \grammarterm{fold-operator} ... ) \grammarterm{fold-operator} E2}