Fix comment and also js build

The python that searches the header files for exports isn't smart
enough to know what is a comment and what isn't, so it picks this
up too and emscripten then complains about it being undefined.

include/olm/pk.h

@@ -152,7 +152,7 @@ size_t olm_pickle_pk_decryption_length(
 /** Stores decryption object as a base64 string. Encrypts the object using the
  * supplied key. Returns the length of the pickled object on success.
  * Returns olm_error() on failure. If the pickle output buffer
- * is smaller than olm_pickle_decryption_length() then
+ * is smaller than olm_pickle_pk_decryption_length() then
  * olm_pk_decryption_last_error() will be "OUTPUT_BUFFER_TOO_SMALL" */
 size_t olm_pickle_pk_decryption(
     OlmPkDecryption * decryption,