nxml-attribute-indent

nxml-attribute-indent is a variable defined in `nxml-mode.el'.
Its value is

4

Documentation:
Indentation for the attributes of an element relative to the start-tag.
This only applies when the first attribute of a tag starts a line.
In other cases, the first attribute on one line is indented the same
as the first attribute on the previous line.

You can customize this variable.