Fix markdown nit (#17163)
authorMartin Pool <mbp@sourcefrog.net>
Thu, 22 Feb 2018 04:13:38 +0000 (20:13 -0800)
committerShanqing Cai <cais@google.com>
Thu, 22 Feb 2018 04:13:38 +0000 (23:13 -0500)
commit3d68260b2c3e48d9a56d4cbb29241e28312375dc
tree20bff86cb675fe89f61ee0951c9a616a6bd1e0f1
parentafae8a8f98b72dd476e7947cda5351fb34bc684f
Fix markdown nit (#17163)

Without a leading blank line, it doesn't render properly in https://www.tensorflow.org/programmers_guide/variables#variable_collections.
tensorflow/docs_src/programmers_guide/variables.md