Typo Fixes - Chapter 7 and Chapter 12

[jeffreygwang]

See title.

[witgaw]

Won't be opening another PR, but I believe line 97 of DimensionalityReduction.tex should be:
||\textbf{x}_{n} - (\textbf{x}_{n} \cdot \textbf{w})\textbf{w}||^{2}