Fix an instance of -Werror=misleading-indentation

Fix an instance of -Werror=misleading-indentation

Full GCC output is:

/home/scott/lyxbuilds/master/repo/src/insets/InsetText.cpp: In member function ‘void lyx::InsetText::docbook(lyx::XMLStream&, const lyx::OutputParams&, lyx::InsetText::XHTMLOptions) const’:
/home/scott/lyxbuilds/master/repo/src/insets/InsetText.cpp:713:5: error: this ‘for’ clause does not guard... [-Werror=misleading-indentation]
  713 |     for (auto const & arg : appendedArguments)
      |     ^~~
/home/scott/lyxbuilds/master/repo/src/insets/InsetText.cpp:717:2: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the ‘for’
  717 |  if (writeOuterTag) {
      |  ^~