script/ceph-backport: Remove stray `\` from grep command

script/ceph-backport: Remove stray `\` from grep command

GNU grep v3.8 release notes[1] has the following mention about the usage
of backslashes:

"Regular expressions with stray backslashes now cause warnings, as their
unspecified behavior can lead to unexpected results."
. . .
"The warnings are intended as a transition aid; they are likely to be
errors in future releases."

As a result we see the warning "grep: warning: stray \ before /" during
script execution. Therefore remove the extra `\` from grep command.

[1] https://git.savannah.gnu.org/cgit/grep.git/tree/NEWS#n85

Signed-off-by: Anoop C S <anoopcs@cryptolab.net>