[SPARK-26928][CORE][FOLLOWUP] Fix JVMCPUSource file name and minor updates to doc

## What changes were proposed in this pull request?

This applies some minor updates/cleaning following up SPARK-26928, notably renaming JVMCPU.scala to JVMCPUSource.scala.

## How was this patch tested?

Manually tested

Closes apache#24201 from LucaCanali/fixupSPARK-26928.

Authored-by: Luca Canali <luca.canali@cern.ch>
Signed-off-by: Sean Owen <sean.owen@databricks.com>

Author: LucaCanali

core/src/main/scala/org/apache/spark/executor/ExecutorSource.scala

@@ -17,12 +17,9 @@
 
 package org.apache.spark.executor
 
-import java.lang.management.ManagementFactory
 import java.util.concurrent.ThreadPoolExecutor
-import javax.management.{MBeanServer, ObjectName}
 
 import scala.collection.JavaConverters._
-import scala.util.control.NonFatal
 
 import com.codahale.metrics.{Gauge, MetricRegistry}
 import org.apache.hadoop.fs.FileSystem

core/src/main/scala/org/apache/spark/metrics/source/JVMCPU.scala renamed to core/src/main/scala/org/apache/spark/metrics/source/JVMCPUSource.scala

@@ -19,7 +19,7 @@ package org.apache.spark.metrics.source
 
 import java.lang.management.ManagementFactory
 
-import com.codahale.metrics.{Counter, Gauge, MetricRegistry}
+import com.codahale.metrics.{Gauge, MetricRegistry}
 import javax.management.{MBeanServer, ObjectName}
 import scala.util.control.NonFatal
 

docs/monitoring.md

@@ -948,7 +948,7 @@ This is the component with the largest amount of instrumented metrics
   - LongAccumulatorSource
 
 - namespace=spark.streaming
-  - **note** applies to Spark Structured Streaming only. Conditional to a configuration
+  - **note:** This applies to Spark Structured Streaming only. Conditional to a configuration
   parameter: `spark.sql.streaming.metricsEnabled=true` (default is false) 
   - eventTime-watermark
   - inputRate-total
@@ -1076,7 +1076,7 @@ Note: applies when running in Spark standalone as worker
 - coresFree
 - memFree_MB
 
-## Component instance = shuffleService
+### Component instance = shuffleService
 Note: applies to the shuffle service
 
 - blockTransferRateBytes (meter)