Fix a typo in BUG message [ci skip]

pack.c

@@ -537,7 +537,7 @@ pack_pack(rb_execution_context_t *ec, VALUE ary, VALUE fmt, VALUE buffer)
                 bigendian_p = explicit_endian == '>';
             }
             if (integer_size > MAX_INTEGER_PACK_SIZE)
-                rb_bug("unexpected intger size for pack: %d", integer_size);
+                rb_bug("unexpected integer size for pack: %d", integer_size);
             while (len-- > 0) {
                 char intbuf[MAX_INTEGER_PACK_SIZE];