Make markdown pretty for some links

Author: HyukjinKwon

core/src/main/scala/org/apache/spark/Accumulator.scala

@@ -26,7 +26,7 @@ package org.apache.spark
  *
  * An accumulator is created from an initial value `v` by calling
  * [[SparkContext#accumulator SparkContext.accumulator]].
- * Tasks running on the cluster can then add to it using the `+=` operator in [[Accumulable]].
+ * Tasks running on the cluster can then add to it using the `+=` operator.
  * However, they cannot read its value. Only the driver program can read the accumulator's value,
  * using its [[#value]] method.
  *

mllib/src/main/scala/org/apache/spark/mllib/fpm/FPGrowth.scala

@@ -309,7 +309,7 @@ object FPGrowth {
 
   /**
    * Frequent itemset.
-   * @param items items in this itemset. Java users should call `FreqItemset#javaItems` instead.
+   * @param items items in this itemset. Java users should call `FreqItemset.javaItems` instead.
    * @param freq frequency
    * @tparam Item item type
    *

mllib/src/main/scala/org/apache/spark/mllib/tree/GradientBoostedTrees.scala

@@ -74,7 +74,7 @@ class GradientBoostedTrees private[spark] (
   }
 
   /**
-   * Java-friendly API for `org.apache.spark.mllib.tree.GradientBoostedTrees!#run`.
+   * Java-friendly API for `org.apache.spark.mllib.tree.GradientBoostedTrees.run`.
    */
   @Since("1.2.0")
   def run(input: JavaRDD[LabeledPoint]): GradientBoostedTreesModel = {
@@ -106,7 +106,7 @@ class GradientBoostedTrees private[spark] (
   }
 
   /**
-   * Java-friendly API for `org.apache.spark.mllib.tree.GradientBoostedTrees!#runWithValidation`.
+   * Java-friendly API for `org.apache.spark.mllib.tree.GradientBoostedTrees.runWithValidation`.
    */
   @Since("1.4.0")
   def runWithValidation(