The ornamental design for the LED Christmas decoration string light, as shown and described.
FIG. 1 is a top plan view of the LED Christmas decoration string light showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a right-side elevational thereof;
FIG. 4 is a back elevational view thereof;
FIG. 5 is a left-side elevational view thereof;
FIG. 6 is a bottom plan view thereof; and
FIG. 7 is a front perspective view thereof; and,
FIG. 8 is an enlarged view of portion 8 shown in FIG. 2.
The broken lines shown in the figures are for the purpose of illustrating portions of the LED Christmas decoration string light, which form no part of the claimed design.