Fix misleading comment.

|num_rounds| is neither a parameter nor manifest constant.

Change-Id: I6c1d3a3819731f53fdd01eef6bb4de8a45176a1d
Reviewed-on: https://boringssl-review.googlesource.com/3180
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/crypto/chacha/chacha_generic.c b/crypto/chacha/chacha_generic.c
index e9fc70e..c5e5003 100644
--- a/crypto/chacha/chacha_generic.c
+++ b/crypto/chacha/chacha_generic.c
@@ -54,7 +54,7 @@
                            size_t counter);
 #endif
 
-/* chacha_core performs |num_rounds| rounds of ChaCha20 on the input words in
+/* chacha_core performs 20 rounds of ChaCha on the input words in
  * |input| and writes the 64 output bytes to |output|. */
 static void chacha_core(uint8_t output[64], const uint32_t input[16]) {
   uint32_t x[16];