mnemonics: typo fix (4 bytes == 3 words, not 8 bytes)

src/mnemonics/electrum-words.cpp

@@ -408,7 +408,7 @@ namespace crypto
       std::vector<std::string> words_store;
 
       uint32_t word_list_length = word_list.size();
-      // 8 bytes -> 3 words.  8 digits base 16 -> 3 digits base 1626
+      // 4 bytes -> 3 words.  8 digits base 16 -> 3 digits base 1626
       for (unsigned int i=0; i < len/4; i++, words += ' ')
       {
         uint32_t w1, w2, w3;