correct the RFC number in the description of the repair option

[alicerusso]

RFC 7966 should be RFC 7996

svgcheck/README.md

Line 41 in d96180b

| `-r` | `--repair` | repair the SVG so it meets RFC 7966, only emit the new file if repairs are needed |

svgcheck/svgcheck/run.py

Line 67 in d96180b

help='Repair the SVG so it meets RFC 7966')