Remove wrong brace

Author: eernstg

specification/dartLangSpec.tex

@@ -16613,7 +16613,6 @@ \subsection{Null Shorting}
 This applies to relational expressions, bitwise expressions,
 shift expressions, additive expressions, multiplicative expressions,
 and unary expressions.
-}
 
 \commentary{%
 For example, \code{$a$ + $b$} is specified as equivalent to