fix typo

[BMDan]

fix typo, clarify wording a bit

[mstorsjo]

While the typo fix in general looks straightforward enough, I'm a bit reluctant to include it.

This isn't an independent project on its own, it's a repackaging of the AAC codec from Android. I include as few differences to the original upstream code as possible, to make sure maintainance of it is manageable - I don't want to diverge functionally from the original. (The differences mainly are in having a different build system, fixing crashes before the fixes get included in the Android version later, fixing other issues that don't matter in the Android context, etc.)

In this case, you could try to get the typos fixed in Android (in the platform/external/aac repo) - in that case the fix will get included here later.