Add errata for some hyphenation inconsistencies

[yousuketakada]

• "linear-Gaussian" vs. "linear Gaussian": Although a quick search on the web reveals that the term "linear Gaussian" (without hyphenation) may be more popular, it seems that the term "linear-Gaussian" is used consistently in PRML.
• "expectation maximization" vs. "expectation-maximization": The term "expectation maximization" (without hyphenation) is better.
• "positive definite" vs. "positive-definite": We say "A is positive definite" but "A is a positive-definite matrix" (we should use hyphenation only before a noun in this case).
• "N-dimensional" vs. "N dimensional": We always use hyphenation if an adjective is formed with an ordinal.

[yousuketakada]

"Sub-sampling layer" on p. 268, Figure 5.17, versus "subsampling layer" on p. 269, e.g. on line 7.