config-linux: Fix 'file' POSIX link
config-linux: Fix 'file' POSIX link

This was broken by f9dc90b0 (make link usage consistent across the
specification, 2017-02-09, #687), which updated the link label, but
not this link.  Now that the link label matches the link text, we can
use the implicit link name shortcut [1].

[1]: https://daringfireball.net/projects/markdown/syntax#link

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

opencontainers/specs,opencontainers/runtime-spec,wking/opencontainer-runtime-spec,hqhq/runtime-spec,hqhq/specs,wking/opencontainer-specs,laijs/specs