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

Before:
```groff
.SH DESCRIPTION
.PP
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.
.SH EXAMPLES
.SS Import from a remote location
```

After:
```groff
.SH DESCRIPTION
.PP
Create a new filesystem image from the contents of a tarball (\f[C]\&.tar\f[],
\f[C]\&.tar.gz\f[], \f[C]\&.tgz\f[], \f[C]\&.bzip\f[], \f[C]\&.tar.xz\f[], \f[C]\&.txz\f[]) into it, then optionally
tag it.
.SH EXAMPLES
.SS Import from a remote location
```