We claim the ornamental design for a routing guide, as shown and described.
FIG. 1 is a first perspective view of a routing guide showing our new design;
FIG. 2 is a rear view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a left side view thereof;
FIG. 7 is a right side view thereof; and,
FIG. 8 is a second perspective view thereof.
The broken lines in the drawings depict unclaimed environmental subject matter, are for illustrative purposes only, and form no part of the claimed design.