Remove an extraneous _ from __COVERITY__

We had a typo in this check, so that coverity wasn't taking the
right path.

Bug not in any released Tor.

src/lib/math/prob_distr.h

@@ -53,7 +53,7 @@ struct dist {
  *  We define this conditionally to suppress false positives from
  *  Coverity, which gets confused by the sizeof business.
  */
-#ifdef __COVERITY___
+#ifdef __COVERITY__
 #define TYPE_CHECK_OBJ(OPS, OBJ, TYPE) 0
 #else
 #define TYPE_CHECK_OBJ(OPS, OBJ, TYPE) \