Pluralize this string even though it's not currently used (Bug 1328760)

The way the code is written, the count passed to "participantsmany" will
always be 2 or more. Therefore, the singular version of this string will
never be used, the way the code is currently written.

However, I think it's a good idea to store it as a proper pluralizable string
anyway, because having it as an array may be necessary in order for it to get
detected as a pluralizable string by the software translators are using. And
it's also possible that future code development could result in the number 1
getting passed to this string.

Change-Id: I87487e2cb64de53b144b19162392b3c0adda860c