reshape(): Remove "nullable" in prose description of newShape

[inexorabletash]

Per #388, the change #505 removed null value support in reshape()'s newShape parameter. That pull request missed updating the prose description to match.

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[inexorabletash]

Trivial fix.

[fdwr]

👍 TY JB.
(@huningxin because this is so trivial, I'll merge it now)

[fdwr]

p.s. I see we have two other places (GRU and BatchNormalization) that will need updating too:

const shape = [1,null,1,1]; <--------------------
return builder.relu(
  builder.add(
    builder.mul(
      builder.reshape(options.scale, shape),
      builder.div(
        builder.sub(input, builder.reshape(mean, shape)),
        builder.sqrt(builder.add(builder.reshape(variance, shape), builder.constant(options.epsilon)))
        )),
    builder.reshape(options.bias, shape))); <--------------------

    currentOutput = builder.reshape(currentOutput, [1, numDirections, null, hiddenSize]); <--------------------

(I can't get to it today, but added local todo)