qcd: Update slash-escapes in change command for Bash 4

I was getting this error when running `qcd`'s change command on a
machine using Bash 4.4.12:

    $ sed -E "s/^shortcut       .+$/shortcut    ${p//\//\/}/" .config/qcd/database
    sed: 1: "s/^shortcut	.+$/shortcut ...": bad flag in substitute command: 'U'

The solution, as described here:
https://stackoverflow.com/questions/27787536/how-to-pass-a-variable-containing-slashes-to-sed/27788661#27788661
is to add an extra backslash to the parameter expansion. This appears to
work on both Bash 3 and 4 in my local manual tests.

0 files changed, 0 insertions, 0 deletions