website: fix volume_type names formatting

for consistent docs pages

Signed-off-by: Mikhail Ushanov <gm.mephisto@gmail.com>

website/source/docs/builders/amazon-chroot.html.md

@@ -195,8 +195,8 @@ each category, the available configuration keys are alphabetized.
     -   `volume_size` (number) - The size of the volume, in GiB. Required if not
         specifying a `snapshot_id`
 
-    -   `volume_type` (string) - The volume type. gp2 for General Purpose (SSD)
-        volumes, io1 for Provisioned IOPS (SSD) volumes, and standard for Magnetic
+    -   `volume_type` (string) - The volume type. `gp2` for General Purpose (SSD)
+        volumes, `io1` for Provisioned IOPS (SSD) volumes, and `standard` for Magnetic
         volumes
 
 -   `region_kms_key_ids` (map of strings) - a map of regions to copy the ami to,