Spec: Fix syntax error in "obtain the plaintext payload"

I was surprised to see errors on CI that appeared unrelated to my
changes in PR #171. I couldn't repro the errors until I upgraded from
bikeshed 4.2.8 to 5.0.0.

For future reference, the errors looked like this prior to fixing the
link syntax:

```
LINE 1126:8: Parsed a dfn autolink ([=...=]) inside of an <a>. Either close the <a> properly, or escape the autolink.
LINE 1127:24: Parsed a dfn autolink ([=...=]) inside of an <a>. Either close the <a> properly, or escape the autolink.
LINE 1124:27: [=...=] autolink opened at 1124:27 wasn't closed within 3 lines. You might have forgotten to close it; if not, switch to the HTML syntax to spread your link across that many lines.
LINE 1124: No 'dfn' refs found for 'filteringid}}"] and'.
[="{{PAHistogramContribution/filteringId}}"] and
=]
```

spec.bs

@@ -1121,7 +1121,7 @@ To <dfn>obtain the plaintext payload</dfn> given an [=aggregatable report=]
             |contribution|["{{PAHistogramContribution/value}}"] and 4.
         : "`id`"
         :: The result of [=encoding an integer for the payload=] given
-            |contribution|[="{{PAHistogramContribution/filteringId}}"] and
+            |contribution|["{{PAHistogramContribution/filteringId}}"] and
             |filteringIdMaxBytes|.
     1. [=list/Append=] |contributionData| to |payloadData|.
 1. Let |payload| be an [=ordered map=] of the following key/value pairs: