doc: don't escape " when not required

[jelly]

This silences a grep warning about a stray \ before ".

Silences:

[jelle@t14s][~/projects/cockpit/main]%make check-guide
if grep -n -r 'name=\"id' doc/guide/html >&2; then \
        echo "Unexpected generated id in the documentation" >&2; \
        exit 1; \
fi
grep: warning: stray \ before "

[martinpitt]

Thanks!