Fix typo in S3 backend expirationSeconds parameter configuration

Documentation specifies this parameter as `expiration` whereas it is
read from configuration as `expirationSeconds`. Update documentation.

Change-Id: Icc0c41f2a7817d87847494d16079a229f2eaf583
Signed-off-by: Jacek Centkowski <geminica.programs@gmail.com>
diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index 602d566..f6ece33 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -124,7 +124,7 @@
 [Amazon S3 storage class] used for storing large objects.
 : Default is `REDUCED_REDUNDANCY`
 
-s3.expiration
+s3.expirationSeconds
 : Expiration in seconds of validity of signed requests. Gerrit's LFS protocol
 handler signs requests to be issued by the git-lfs extension with the configured
 `accessKey` and `secretKey`. This way the git-lfs extension doesn't need