Updating the writeup of the RNN design doc and sequence_decoder

[kavyasrinet]

Few comments:

1. Since I could not edit the image(https://github.com/PaddlePaddle/Paddle/blob/develop/doc/design/ops/images/2_level_rnn.png ) attached:
   We should update the following:
   2-th -> 2-nd/ 2nd/ second
   1-th -> 1-st / 1st/ first
2. We also need to edit the hyperlinks that point to @Superjom 's repo in the Decoder paragraph in this section: https://github.com/PaddlePaddle/Paddle/blob/develop/doc/design/ops/sequence_decoder.md#usage-in-a-simple-machine-translation-model

[Superjomn]

Thank you for fixing these mistakes.

for

We should update the following:
2-th -> 2-nd/ 2nd/ second
1-th -> 1-st / 1st/ first

all the PNGs should have a corresponding dot file, that image's dot locates at
https://github.com/PaddlePaddle/Paddle/blob/develop/doc/design/ops/images/2_level_rnn.dot

you can also change it in this PR.

[kavyasrinet]

Thank you for pointing me to the file. Will update the file in this PR.

[kavyasrinet]

@Superjom Fixed the dot file, please review when you can.

[Superjomn]

LGTM