Finish an incomplete sentence in a proposal.

doc/spec/proposals/158-microdescriptors.txt

@@ -122,7 +122,7 @@ Status: Open
     http://<hostname>/tor/micro/d/<D1>-<D2>-<D3>.z
   (We use base64 for size and for consistency with the consensus
   format. We use -s instead of +s to separate these items, since
-  ... since...?
+  the + character use used in base64 encoding.)
 
   All the microdescriptors from the current consensus should also be
   available at: