The ornamental design for a substrate with a pattern, as shown and described.
FIG. 1 is a perspective view of a substrate with a pattern;
FIG. 2 is a front view of a substrate with a pattern
FIG. 3 is a left side view of a substrate with a pattern;
FIG. 4 is a rear view of a substrate with a pattern;
FIG. 5 is a right side view of a substrate with a pattern;
FIG. 6 is a top view of a substrate with a pattern; and,
FIG. 7 is a bottom view of a substrate with a pattern.
The dot-dash-dot broken lines immediately adjacent to the solid lines in the drawings depict the bounds of the claim and form no part thereof. All other broken lines represent environmental subject matter that forms no part of the claimed design.