Squishing a typo bug before it causes real harm

in updateNumRows method in RowMatrix

Author: Doris Xin <doris.s.xin@gmail.com>

Closes apache#1125 from dorx/updateNumRows and squashes the following commits:

8564aef [Doris Xin] Squishing a typo bug before it causes real harm

mllib/src/main/scala/org/apache/spark/mllib/linalg/distributed/RowMatrix.scala

@@ -419,7 +419,7 @@ class RowMatrix(
   /** Updates or verifies the number of rows. */
   private def updateNumRows(m: Long) {
     if (nRows <= 0) {
-      nRows == m
+      nRows = m
     } else {
       require(nRows == m,
         s"The number of rows $m is different from what specified or previously computed: ${nRows}.")