Update StreamingKMeans.scala

## What changes were proposed in this pull request?

I think the ‘n_t+t’ in the following code may be wrong, it shoud be ‘n_t+1’ that means  is the number of points to the cluster  after it finish the no.t+1 min-batch.

 * <blockquote>
 *    $$
 *    \begin{align}
 *     c_t+1 &= [(c_t * n_t * a) + (x_t * m_t)] / [n_t + m_t] \\
 *     n_t+t &= n_t * a + m_t
 *    \end{align}
 *    $$
 * </blockquote>

Author: Fan Donglai <[email protected]>

Closes apache#21179 from ddna1021/master.

Author: ddna1021

mllib/src/main/scala/org/apache/spark/mllib/clustering/StreamingKMeans.scala

@@ -43,7 +43,7 @@ import org.apache.spark.util.random.XORShiftRandom
  *    $$
  *    \begin{align}
  *     c_t+1 &= [(c_t * n_t * a) + (x_t * m_t)] / [n_t + m_t] \\
- *     n_t+t &= n_t * a + m_t
+ *     n_t+1 &= n_t * a + m_t
  *    \end{align}
  *    $$
  * </blockquote>