docs: use correct storage type constant in docstrings (#110)

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [ ] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/python-bigtable/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

google/cloud/bigtable/cluster.py

@@ -59,7 +59,7 @@ class Cluster(object):
                                  Possible values are represented by the
                                  following constants:
                                  :data:`google.cloud.bigtable.enums.StorageType.SSD`.
-                                 :data:`google.cloud.bigtable.enums.StorageType.SHD`,
+                                 :data:`google.cloud.bigtable.enums.StorageType.HDD`,
                                  Defaults to
                                  :data:`google.cloud.bigtable.enums.StorageType.UNSPECIFIED`.
 

google/cloud/bigtable/instance.py

@@ -268,7 +268,7 @@ def create(
                                       Possible values are represented
                                       by the following constants:
                                       :data:`google.cloud.bigtable.enums.StorageType.SSD`.
-                                      :data:`google.cloud.bigtable.enums.StorageType.SHD`,
+                                      :data:`google.cloud.bigtable.enums.StorageType.HDD`,
                                       Defaults to
                                       :data:`google.cloud.bigtable.enums.StorageType.UNSPECIFIED`.
 
@@ -564,7 +564,7 @@ def cluster(
                                      Possible values are represented by the
                                      following constants:
                                      :data:`google.cloud.bigtable.enums.StorageType.SSD`.
-                                     :data:`google.cloud.bigtable.enums.StorageType.SHD`,
+                                     :data:`google.cloud.bigtable.enums.StorageType.HDD`,
                                      Defaults to
                                      :data:`google.cloud.bigtable.enums.StorageType.UNSPECIFIED`.