[dcl.struct.bind] Fix tuple-like binding index to use SB_i instead of v_i

P1061R10 introduced the SB_i notation for post-expansion structured bindings
but this sentence was not updated. The rest of the section (p6, p8, and the
end of this same paragraph) already uses SB_i.

Author: abhinavagarwal07

source/declarations.tex

@@ -7724,7 +7724,7 @@
 whose value is non-negative;
 the structured binding size of \tcode{E} is equal to that value.
 Let \tcode{i} be an index prvalue of type \tcode{std::size_t}
-corresponding to $\tcode{v}_i$.
+corresponding to $\textrm{SB}_i$.
 If a search for the name \tcode{get}
 in the scope of \tcode{E}\iref{class.member.lookup}
 finds at least one declaration