[dcl.type.simple] Fix excess vertical whitespace before grammar

excerpt.

Author: zygoloid

source/declarations.tex

@@ -1385,9 +1385,11 @@
 the \grammarterm{template-name} shall name a deducible template.
 A \defnadj{deducible}{template} is either a class template or
 is an alias template whose \grammarterm{defining-type-id} is of the form
+
 \begin{ncsimplebnf}
 \opt{\keyword{typename}} \opt{nested-name-specifier} \opt{\keyword{template}} simple-template-id
 \end{ncsimplebnf}
+
 where the \grammarterm{nested-name-specifier} (if any) is non-dependent and
 the \grammarterm{template-name} of the \grammarterm{simple-template-id}
 names a deducible template.