gptar-update.md: Add shell language

articles/gptar-update.md

@@ -29,7 +29,7 @@ This prompted me to [use a different link indicator](https://github.com/reynir/g
 This worked pretty great.
 Listing the archive using GNU tar I still get `GPTAR`, but with verbose listing it's displayed as a `--Volume Header--`:
 
-```
+```shell
 $ tar -tvf disk.img
 Vr-------- 0/0           16896 1970-01-01 01:00 GPTAR--Volume Header--
 -rw-r--r-- 0/0              14 1970-01-01 01:00 test.txt
@@ -37,7 +37,7 @@ Vr-------- 0/0           16896 1970-01-01 01:00 GPTAR--Volume Header--
 
 And more importantly the `GPTAR` entry is ignored when extracting:
 
-```
+```shell
 $ mkdir tmp
 $ cd tmp/
 $ tar -xf ../disk.img
@@ -49,7 +49,7 @@ test.txt
 
 Unfortunately, this broke bsdtar!
 
-```
+```shell
 $ bsdtar -tf disk.img
 bsdtar: Damaged tar archive
 bsdtar: Error exit delayed from previous errors.
@@ -62,7 +62,7 @@ Unacceptable!
 So I started to dig into libarchive where bsdtar comes from.
 To my surprise, after building bsdtar from the git clone of the source code it ran perfectly fine!
 
-```
+```shell
 $ ./bsdtar -tf ../gptar/disk.img
 test.txt
 ```