DOC: Add missing word in docstring

Author: Robert Gieseke <[email protected]>

Closes pandas-dev#13915 from rgieseke/patch-2 and squashes the following commits:

bbdcdea [Robert Gieseke] DOC: Add missing word

Author: rgieseke

pandas/io/pytables.py

@@ -285,8 +285,8 @@ def read_hdf(path_or_buf, key=None, **kwargs):
 
             .. versionadded:: 0.19.0 support for pathlib, py.path.
 
-        key : group identifier in the store. Can be omitted a HDF file contains
-            a single pandas object.
+        key : group identifier in the store. Can be omitted if the HDF file
+            contains a single pandas object.
         where : list of Term (or convertable) objects, optional
         start : optional, integer (defaults to None), row number to start
             selection