SSD document clarity.

[NorthStar]

Due to pre-commit issue, now I am using a new branch to replace #304

Attempts to fix #4246 from Paddle: PaddlePaddle/Paddle#4246 , one round of review for English.

Note that the diagram says mAP for mean Value Precision, but when I look things up, they are all just MAP, so I corrected accordingly.

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