[css-nav-1] Fix spurious / in <img>

[alkorlos]

Fix bikeshed errors.

The command:

bikeshed spec css-nav-1\Overview.bs

Throws errors:

LINE 137:5: Spurious / in <img>.
LINE 278:9: Spurious / in <img>.
LINE 279:9: Spurious / in <img>.
LINE 288:9: Spurious / in <img>.
LINE 289:9: Spurious / in <img>.
LINE 290:9: Spurious / in <img>.
LINE 291:9: Spurious / in <img>.
LINE 554:9: Spurious / in <img>.
LINE 785:9: Spurious / in <img>.
LINE 841:9: Spurious / in <img>.
LINE 1311:7: Spurious / in <img>.
LINE 1528:9: Spurious / in <img>.
LINE 1606:9: Spurious / in <img>.