[range.split.iterator] Add missing braces in 'Effects' clause (#4721)

Author: cpplearner

source/ranges.tex

@@ -6374,7 +6374,7 @@
 \begin{itemdescr}
 \pnum
 \effects
-Equivalent to \tcode{return {\exposid{cur_}, \exposid{next_}.begin()};}
+Equivalent to \tcode{return \{\exposid{cur_}, \exposid{next_}.begin()\};}
 \end{itemdescr}
 
 \begin{itemdecl}