I claim the ornamental design for a rain gutter cover, as shown and described.
FIG. 1 is a perspective view of a rain gutter cover, showing my new design;
FIG. 2 is a left-side elevational view thereof, the right-side elevational view being a mirror image thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a front elevational view thereof; and,
FIG. 6 is a rear elevational view thereof.
The broken lines showing a roof and rain gutter in FIG. 1 depict environmental matter and form no part of the claimed design. The gutter cover is shown broken away in the figures to indicate that no particular length of gutter cover forms a part of the claimed design.