correct the wrong comment for encodeZigZag

Summary:
The comment for encodeZigZag(x) is wrong.

Should be encodeZigZag(x) == -2*x - 1, as

encodeZigZag(-1) = 1

and

encodeZigZag(-2) = 3

Closes facebook/folly#2046

X-link: facebook/folly#2072

Reviewed By: Gownta

Differential Revision: D58789263

Pulled By: yfeldblum

fbshipit-source-id: af4d16b606cdc39ec930f16218ebf53637a86815

third-party/folly/src/folly/Varint.h

@@ -90,7 +90,7 @@ Expected<uint64_t, DecodeVarintError> tryDecodeVarint(Range<T*>& data);
  * encoding negative values using Varint would use up 9 or 10 bytes.
  *
  * if x >= 0, encodeZigZag(x) == 2*x
- * if x <  0, encodeZigZag(x) == -2*x + 1
+ * if x <  0, encodeZigZag(x) == -2*x - 1
  */
 
 inline uint64_t encodeZigZag(int64_t val) {