Typo in MPSMatrixMultiplication comment (#622)

Author: jandrej

lib/mps/matrix.jl

@@ -135,7 +135,7 @@ end
     matmul!(a::MtlMatrix, b::MtlMatrix, c::MtlMatrix, alpha=1, beta=1,
               transpose_left=false, transpose_right=false)
 A `MPSMatrixMultiplication` kernel that computes:
-`c = alpha * op(a) * beta * op(b) + beta * C`
+`c = alpha * op(a) * op(b) + beta * C`
 
 This function should not typically be used. Rather, use the normal `LinearAlgebra` interface
 with any `MtlArray` and it should be accelerated using Metal Performance Shaders.