manual: Use SYNOPSIS instead of USAGE in man/runc-checkpoint.8.md

Reason1:
 other runc manual are using word of "SYNOPSIS"
 # grep USAGE man/*
 # man/runc-checkpoint.8.md:#USAGE
 #
 # grep SYNOPSIS man/*
 man/runc-checkpoint.8.md:# SYNOPSIS
 man/runc-delete.8.md:# SYNOPSIS
 man/runc-events.8.md:# SYNOPSIS
 man/runc-exec.8.md:# SYNOPSIS
 man/runc-kill.8.md:# SYNOPSIS
 man/runc-list.8.md:# SYNOPSIS
 man/runc-pause.8.md:# SYNOPSIS
 man/runc-restore.8.md:# SYNOPSIS
 man/runc-resume.8.md:# SYNOPSIS
 man/runc-spec.8.md:# SYNOPSIS
 man/runc-start.8.md:# SYNOPSIS
 man/runc-state.8.md:# SYNOPSIS
 man/runc-update.8.md:# SYNOPSIS
 man/runc.8.md:# SYNOPSIS
 #

Reason2:
 Most linux commands are using word of "SYNOPSIS"
 # man ls
 # man find
 # ...

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>

man/runc-checkpoint.8.md

@@ -1,7 +1,7 @@
 # NAME
    runc checkpoint - checkpoint a running container
 
-# USAGE
+# SYNOPSIS
    runc checkpoint [command options] <container-id>
 
 Where "<container-id>" is the name for the instance of the container to be