Fix some warnings due to backslashes in docstrings

This fixes the following warning that appears when you run
`python setup.py test`:

```
=============================== warnings summary ===============================
ligo/gracedb/rest.py:1203
  /Users/lpsinger/src/gracedb-client/ligo/gracedb/rest.py:1203: DeprecationWarning: invalid escape sequence \
    """  # noqa: W605

ligo/gracedb/rest.py:1300
  /Users/lpsinger/src/gracedb-client/ligo/gracedb/rest.py:1300: DeprecationWarning: invalid escape sequence \
    """  # noqa: W605

ligo/gracedb/rest.py:2227
  /Users/lpsinger/src/gracedb-client/ligo/gracedb/rest.py:2227: DeprecationWarning: invalid escape sequence \
    """  # noqa: W605

-- Docs: https://docs.pytest.org/en/latest/warnings.html
================ 488 passed, 98 deselected, 3 warnings in 5.63s ================
```
10 jobs for fix-backslash-warnings in 8 minutes and 32 seconds (queued for 7 minutes and 38 seconds)
Status Job ID Name Coverage
  Build
passed #557953
build:debian:jessie

00:04:06

passed #557954
build:debian:stretch

00:01:41

passed #557952
build:el7

00:01:15

passed #557951
build:tarball

00:00:28

 
  Test
passed #557959
docs

00:00:56

passed #557960
syntax

00:00:37

passed #557955
test:python2.7

00:00:45

passed #557956
test:python3.5

00:00:35

passed #557957
test:python3.6

00:00:35

passed #557958
test:python3.7

00:00:36