[SPARK-27467][FOLLOW-UP][BUILD] Upgrade Maven to 3.6.1 in AppVeyor an…

…d Doc

## What changes were proposed in this pull request?

Update the `docs/building-spark.md`. Otherwise:
```
mvn package -DskipTests=true
...
[INFO] --- maven-enforcer-plugin:3.0.0-M2:enforce (enforce-versions)  spark-parent_2.12 ---
[WARNING] Rule 0: org.apache.maven.plugins.enforcer.RequireMavenVersion failed with message:
Detected Maven Version: 3.6.0 is not in the allowed range 3.6.1.
...
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-enforcer-plugin:3.0.0-M2:enforce (enforce-versions) on project spark-parent_2.12: Some Enforcer rules have failed. Look above for specific messages explaining why the rule failed. -> [Help 1]
[ERROR]
...
```

## How was this patch tested?
Just test `https://archive.apache.org/dist/maven/maven-3/3.6.1/binaries/apache-maven-3.6.1-bin.zip` is avilable.

Closes #24477 from wangyum/SPARK-27467.

Authored-by: Yuming Wang <yumwang@ebay.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

dev/appveyor-install-dependencies.ps1

@@ -81,7 +81,7 @@ if (!(Test-Path $tools)) {
 # ========================== Maven
 Push-Location $tools
 
-$mavenVer = "3.6.0"
+$mavenVer = "3.6.1"
 Start-FileDownload "https://archive.apache.org/dist/maven/maven-3/$mavenVer/binaries/apache-maven-$mavenVer-bin.zip" "maven.zip"
 
 # extract

docs/building-spark.md

@@ -27,7 +27,7 @@ license: |
 ## Apache Maven
 
 The Maven-based build is the build of reference for Apache Spark.
-Building Spark using Maven requires Maven 3.6.0 and Java 8.
+Building Spark using Maven requires Maven 3.6.1 and Java 8.
 Spark requires Scala 2.12; support for Scala 2.11 was removed in Spark 3.0.0.
 
 ### Setting up Maven's Memory Usage