[gram.dcl] Extra line between using-declaration and using-directive

[AndrewPaxie]

In N4431 [gram.dcl] there is one too many lines between the
using-declaration and using-directive nonterminals.

The cause of this formatting error is a leftover \br in the one remaining
production of using-declaration.

Suggested fix:

  \begin{bnf}
  \nontermdef{using-declaration}\br
-     \terminal{using typename\opt} nested-name-specifier unqualified-id \terminal{;}\br
+     \terminal{using typename\opt} nested-name-specifier unqualified-id \terminal{;}
  \end{bnf}