Update tail docs to match built-in docs (docker#2253)

Default value for tail helps me remember it's not a switch, but requires a value. This change makes it match the built-in docs.

Author: ryansanford

compose/reference/logs.md

@@ -12,7 +12,7 @@ Options:
 --no-color          Produce monochrome output.
 -f, --follow        Follow log output
 -t, --timestamps    Show timestamps
---tail              Number of lines to show from the end of the logs
+--tail="all"        Number of lines to show from the end of the logs
                     for each container.
 ```