Fix scaladoc for UInt.unary_!

It performs the operation (x === 0.U), just like in C.   The scaladoc
incorrectly described it as performing the operation !x(0).  (Obviously, these
are equivalent for Bool, but not for UInt in general).

chiselFrontend/src/main/scala/chisel3/core/Bits.scala

@@ -848,7 +848,7 @@ sealed class UInt private[core] (width: Width) extends Bits(width) with Num[UInt
 
   /** Unary not
     *
-    * @return a hardware [[Bool]] asserted if the least significant bit of this $coll is zero
+    * @return a hardware [[Bool]] asserted if this $coll equals zero
     * @group Bitwise
     */
   final def unary_! () : Bool = macro SourceInfoTransform.noArg