Minor handbook fixes for Pricing Model page

• Fixed a typo: "when calculating margings" -> "when calculating margins"
• changed reference to CEO "my presentation" -> "Sid's presentation". Even though the page is in the /ceo/ directory, it might be confusing to the reader, as opposed to, for example, a blog post signed by a specific person. It's now consistent with the rest of this page, where Sid is referred to in the third person.