remove the unexpected 'a'

Author: ShuhuaGao

test/tst_mnist.jl

@@ -49,7 +49,7 @@ end
     A = MNIST.convert2image(data; black_digits=true)
     @test A[1] == 1
     @test A[3, 3, 2] == 0.6
-    @test size(A) == (28,28,2)a
+    @test size(A) == (28,28,2)
     @test eltype(A) == Gray{N0f8}
     @test MNIST.convert2image(vec(data); black_digits=true) == A 
 end