[dcl.contract.func] Add missing \br in grammar (#7838)

Author: OndrejPopp

source/declarations.tex

@@ -4486,7 +4486,7 @@
 \end{bnf}
 
 \begin{bnf}
-\nontermdef{precondition-specifier}
+\nontermdef{precondition-specifier}\br
   \terminal{pre} \opt{attribute-specifier-seq} \terminal{(} conditional-expression \terminal{)}
 \end{bnf}