Fix contrib/man/md/docker-import.1.md warning ("macro `tar.gz,' not defined")

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)

contrib/man/md/docker-import.1.md

@@ -9,8 +9,8 @@ of the tarball into it.
 **docker import** URL|- [REPOSITORY[:TAG]]
 
 # DESCRIPTION
-Create a new filesystem image from the contents of a tarball (.tar,
-.tar.gz, .tgz, .bzip, .tar.xz, .txz) into it, then optionally tag it.
+Create a new filesystem image from the contents of a tarball (`.tar`,
+`.tar.gz`, `.tgz`, `.bzip`, `.tar.xz`, `.txz`) into it, then optionally tag it.
 
 # EXAMPLES