Fixing docstrings that don't end with a period
Fix the following docstrings and make them end with a period to follow the pandas convention.

```
pandas.DataFrame.keys: Summary does not end with a period
pandas.read_clipboard: Summary does not end with a period
pandas.ExcelFile.parse: Summary does not end with a period
pandas.HDFStore.put: Summary does not end with a period
pandas.HDFStore.get: Summary does not end with a period
pandas.HDFStore.select: Summary does not end with a period
pandas.HDFStore.keys: Summary does not end with a period
pandas.HDFStore.groups: Summary does not end with a period
pandas.HDFStore.walk: Summary does not end with a period
pandas.io.stata.StataReader.data: Summary does not end with a period
pandas.io.formats.style.Styler.loader: Summary does not end with a period
pandas.io.formats.style.Styler.set_caption: Summary does not end with a period
pandas.plotting.deregister_matplotlib_converters: Summary does not end with a period
pandas.plotting.register_matplotlib_converters: Summary does not end with a period
```
Issue noted by @datapythonista 