Ruleset: various minor improvements (sirbrillig#244)

* Add XML schema references.
* Remove the sniff reference. A Ruleset for a registered standard will automatically include all sniffs in that standard, so no need to explicitly include the sniff.
* Consistently use four space indent.

Co-authored-by: jrfnl <jrfnl@users.noreply.github.com>

Author: jrfnl

VariableAnalysis/ruleset.xml

@@ -1,5 +1,4 @@
 <?xml version="1.0"?>
-<ruleset name="VariableAnalysis">
-  <description>Plugin for PHP_CodeSniffer static analysis tool that adds analysis of problematic variable use.</description>
-   <rule ref="VariableAnalysis.CodeAnalysis.VariableAnalysis"/>
+<ruleset xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" name="VariableAnalysis" xsi:noNamespaceSchemaLocation="https://raw.githubusercontent.com/squizlabs/PHP_CodeSniffer/master/phpcs.xsd">
+    <description>Plugin for PHP_CodeSniffer static analysis tool that adds analysis of problematic variable use.</description>
 </ruleset>