Update documentation around region lookup behavior (#361)

The lookup uses a label, not an annotation, for determining the region if one is not provided.

Co-authored-by: Simon Marty <simon.marty0@gmail.com>

README.md

@@ -110,7 +110,7 @@ The parameters section contains the details of the mount request and contain one
             - objectName: "MySecret"
               objectType: "secretsmanager"
     ```
-* region: An optional field to specify the AWS region to use when retrieving secrets from Secrets Manager or Parameter Store. If this field is missing, the provider will lookup the region from the annotation on the node. This lookup adds overhead to mount requests so clusters using large numbers of pods will benefit from providing the region here.
+* region: An optional field to specify the AWS region to use when retrieving secrets from Secrets Manager or Parameter Store. If this field is missing, the provider will lookup the region from the `topology.kubernetes.io/region` label on the node. This lookup adds overhead to mount requests so clusters using large numbers of pods will benefit from providing the region here.
 * failoverRegion: An optional field to specify a secondary AWS region to use when retrieving secrets. See the Automated Failover Regions section in this readme for more information.
 * pathTranslation: An optional field to specify a substitution character to use when the path separator character (slash on Linux) is used in the file name. If a Secret or parameter name contains the path separator failures will occur when the provider tries to create a mounted file using the name. When not specified the underscore character is used, thus My/Path/Secret will be mounted as My_Path_Secret. This pathTranslation value can either be the string "False" or a single character string. When set to "False", no character substitution is performed.