Fix missing space in XML header

[renkun-ken]

XML validator complains the missing space in the XML header produced by xml_parse_data.

<?xml version="1.0" encoding="UTF-8"standalone="yes" ?>

should be

<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>

[codecov-io]

Codecov Report

Merging #7 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master     #7   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           2      2           
  Lines          56     56           
=====================================
  Hits           56     56

Impacted Files	Coverage Δ
R/package.R	100% <100%> (ø)	⬆️

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[gaborcsardi]

Thanks!