Small Typo about Encoder Mapping

**Location**
Please provide:
1. Version: Online draft version July 8, 2022, http://smlbook.org
2. Page number: 277
3. Line/equation number: Line 6

**Describe the mistake**
"That is, an encoder mapping z = ℎ_theta (z) which takes a
data point x"

**Suggested change**
Change the z in ℎ_theta (z) to x.

**Additional comments**
