Update proposals/simd/SIMD.md

Co-authored-by: Thomas Lively <7121787+tlively@users.noreply.github.com>

Author: Maratyszcza

proposals/simd/SIMD.md

@@ -413,7 +413,7 @@ def S.neg(a):
 
 ## Extended integer arithmetic
 
-### Extended integer multiplication
+### Extended pairwise integer addition
 * `i16x8.extadd_pairwise_i8x16_s(a: v128) -> v128`
 * `i16x8.extadd_pairwise_i8x16_u(a: v128) -> v128`
 * `i32x4.extadd_pairwise_i16x8_s(a: v128) -> v128`