DOC: Typos in pandas.core.groupby.groupby.GroupBy.rank

#### Problem description
Typos in [pandas.core.groupby.groupby.GroupBy.rank](https://github.com/pandas-dev/pandas/blob/5edc5c4acde1f0d05b598825ab5c6a86fe551484/pandas/core/groupby/groupby.py)
https://github.com/pandas-dev/pandas/blob/5edc5c4acde1f0d05b598825ab5c6a86fe551484/pandas/core/groupby/groupby.py#L1848
'default'
https://github.com/pandas-dev/pandas/blob/5edc5c4acde1f0d05b598825ab5c6a86fe551484/pandas/core/groupby/groupby.py#L1854
‘method’ is mentioned twice

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: acc89dbdb4d469669b1581ba5dd9892bc177c836
python: 3.6.3.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en
LOCALE: None.None

pandas: 0.23.0.dev0+741.gacc89dbdb
pytest: 3.2.3
pip: 9.0.1
setuptools: 36.5.0.post20170921
Cython: 0.27.2
numpy: 1.13.3
scipy: 0.19.1
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.1.0
openpyxl: 2.5.0b1
xlrd: 1.0.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.1
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: 1.1.13
pymysql: 0.7.11.None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
