fix markup error

Doc/whatsnew/whatsnew25.tex

@@ -699,7 +699,7 @@ Brett Cannon and Guido van Rossum; implemented by Brett Cannon.}
 
 The NumPy developers had a problem that could only be solved by adding
 a new special method, \method{__index__}.  When using slice notation,
-as in \code{[\var{start}:\var{stop}:\var{step}], the values of the
+as in \code{[\var{start}:\var{stop}:\var{step}]}, the values of the
 \var{start}, \var{stop}, and \var{step} indexes must all be either
 integers or long integers.  NumPy defines a variety of specialized
 integer types corresponding to unsigned and signed integers of 8, 16,