nixpkgs/doc/stdenv: fix admonition class

Change to match the formatting of the other admonitions that use
classes.

This was breaking the mmdoc output.

Changed files
+1 -1
doc
+1 -1
doc/stdenv/stdenv.chapter.md
···
Use `--prefix` to explicitly set dependencies in `PATH`.
-
:::{note}
+
::: {.note}
`--prefix` essentially hard-codes dependencies into the wrapper.
They cannot be overridden without rebuilding the package.
:::