Fix mqwebuser.xml example by adding root server element

The example `mqwebuser.xml` is currently incorrect, and causes an XML parsing FDC in Liberty.  The example needs a `server` root element.

Author: arthurbarr

charts/ibm-mq/README.md

@@ -389,7 +389,10 @@ metadata:
   name: mywebconfig
 data:
   mqwebuser.xml: |-
-    <variable name="myCustomVariable" value="*"/>
+    <?xml version="1.0" encoding="UTF-8"?>
+    <server>
+      <variable name="myCustomVariable" value="*"/>
+    </server>
 ```
 
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