Fix pre-release matching regex character class

Previously matched any single character from `abc|r` (with duplicate
specification of `c`).  Now matches any single character from `abc` or
the two character sequence `rc`.

I guess this was the intent, anyway.

src/allmydata/util/verlib.py

@@ -254,7 +254,7 @@ def suggest_normalized_version(s):
     # if we have something like "b-2" or "a.2" at the end of the
     # version, that is pobably beta, alpha, etc
     # let's remove the dash or dot
-    rs = re.sub(r"([abc|rc])[\-\.](\d+)$", r"\1\2", rs)
+    rs = re.sub(r"([abc]|rc)[\-\.](\d+)$", r"\1\2", rs)
 
     # 1.0-dev-r371 -> 1.0.dev371
     # 0.1-dev-r79 -> 0.1.dev79
@@ -324,4 +324,3 @@ def suggest_normalized_version(s):
     except IrrationalVersionError:
         pass
     return None
-