Fix invalid XML declaration in getConfigXML() (#71)

[fjallemark]

Fixes the malformed XML declaration emitted by getConfigXML() in
software/esp-firmware/wifiHandling.cpp, reported in #71.

The current declaration has two problems:

String resp = String("<?XML version=\"1.0\" encoding=\"UTF?8\"?>\r\n")

1. <?XML (uppercase) is not a valid XML declaration. Per W3C XML 1.0
   §2.8 the literal is lowercase <?xml, and §2.6 explicitly reserves any
   case variation of "xml" as a processing-instruction target — so the
   output is neither a valid declaration nor a valid PI, and strict parsers
   (e.g. .NET XDocument.Parse) reject it.
2. encoding="UTF?8" is not a valid encoding name — the - appears to have
   been corrupted into a ?. It should be UTF-8.

This changes only that one string literal to a spec-compliant declaration:

String resp = String("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\r\n")

No other behavior changes; the element/attribute structure is untouched.

I understand from #71 that you don't maintain the XML config feature — this
is offered purely as a minimal correctness fix so the output parses with
standards-compliant XML parsers without a client-side workaround. Feel free
to take or leave it.

[pischky]

I would prefer that the XMLDecl would be fixed.