amsmath's intertext macro is used, but package not included

[andreaswachowski]

On the book's website, section "4.1.7.1 Derivation of Marginal Distribution for μ" (see https://statswithr.github.io/book/inference-and-decision-making-with-multiple-parameters.html#sec:normal-gamma) shows "\intertext" in red, and the included text is cropped.

Note: The amsmath package was once in preamble.tex, but commit 600c9db removed it.