Update matrix_multiplication.md

There is a typo on the example

Author: el-oso

docs/src/examples/matrix_multiplication.md

@@ -9,7 +9,7 @@ function A_mul_B!(C, A, B)
     @avx for n ∈ indices((C,B), 2), m ∈ indices((C,A), 1)
         Cmn = zero(eltype(C))
         for k ∈ indices((A,B), (2,1))
-            Cmn += C[m,k] * B[k,n]
+            Cmn += A[m,k] * B[k,n]
         end
         C[m,n] = Cmn
     end