Clarify sentence about "last_affected" and "fixed" in docs.

[tiegz]

From the docs:

Entries in the events array can contain either "last_affected" or "fixed" events, but not both.

This line could be referring to either of these events arrays:

{
  "events": [
    {"introduced": "0.0.0"},
    {"last_affected": "1.0.0", "fixed": "1.0.1"}
  ]
}

{
  "events": [
    {"introduced": "0.0.0"},
    {"last_affected": "1.0.0"},
    {"fixed": "1.0.1"}
  ]
}

The first one is invalid because it has an Object that contains two entries, but that is covered by this line from the docs:

Only a single type (either introduced, fixed, last_affected, limit) is allowed in each event object.

And according to this comment I think the latter invalid case is the one that is being referred to: #146 (comment)

So I suggest changing the wording to be more specific:

An events array can have entries containing either "last_affected" or "fixed" events, but not both.