Fix typo in sample code of ch15, layers API

[hanocha]

Dear author,

Hi :)
I found mistakes in chapter 15, sample codes of Tensorflow layers API.

The sample code with validation_set, it outputs results of validation almost 1.00, it seems strange to me.
Maybe we should use validation_set argument in the block of if.

And this PR includes some of fixes about white spaces of the end of line, and blank lines.
It was auto corrected when I saved the file.
If you don't line the differences, please mention to me. I'll modify it.

[pep8speaks]

Hello @hanocha! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file code/ch15/ch15.py:

Line 333:1: E266 too many leading '#' for block comment
Line 340:9: E266 too many leading '#' for block comment
Line 456:35: E231 missing whitespace after ','
Line 459:5: E266 too many leading '#' for block comment
Line 604:1: E266 too many leading '#' for block comment
Line 620:1: E266 too many leading '#' for block comment
Line 641:1: E266 too many leading '#' for block comment
Line 652:1: E266 too many leading '#' for block comment
Line 669:1: E266 too many leading '#' for block comment
Line 690:1: E266 too many leading '#' for block comment
Line 749:5: E266 too many leading '#' for block comment
Line 850:9: E266 too many leading '#' for block comment
Line 866:31: E128 continuation line under-indented for visual indent
Line 869:54: E231 missing whitespace after ','
Line 872:9: E266 too many leading '#' for block comment
Line 874:31: E128 continuation line under-indented for visual indent
Line 946:24: E222 multiple spaces after operator

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