[CORE][DOC] Fix errors in comments

## What changes were proposed in this pull request?
While reading source code of CORE and SQL core, I found some minor errors in comments such as extra space, missing blank line and grammar error.

I fixed these minor errors and might find more during my source code study.

## How was this patch tested?
Manually build

Author: wm624@hotmail.com <wm624@hotmail.com>

Closes #15151 from wangmiao1981/mem.

core/src/main/scala/org/apache/spark/storage/BlockManagerId.scala

@@ -28,7 +28,7 @@ import org.apache.spark.util.Utils
  * :: DeveloperApi ::
  * This class represent an unique identifier for a BlockManager.
  *
- * The first 2 constructors of this class is made private to ensure that BlockManagerId objects
+ * The first 2 constructors of this class are made private to ensure that BlockManagerId objects
  * can be created only using the apply method in the companion object. This allows de-duplication
  * of ID objects. Also, constructor parameters are private to ensure that parameters cannot be
  * modified from outside this class.

sql/core/src/main/scala/org/apache/spark/sql/SparkSession.scala

@@ -937,7 +937,7 @@ object SparkSession {
   }
 
   /**
-   * Return true if Hive classes can be loaded, otherwise false.
+   * @return true if Hive classes can be loaded, otherwise false.
    */
   private[spark] def hiveClassesArePresent: Boolean = {
     try {