acoustics: fix

There's a bit of LaTeX math code in a comment in a test file:

```
    .. math:: L = 10 \cdot \\log_{10}{\\left(\\frac{MS}{p_r^2} \\right)}
```

which of course should be `\\cdot`.

I could patch it, but I think skipping deprecation warnings in tests is
probably ok... (If nothing else it's easier.)

pkgs/development/python-modules/acoustics/default.nix

@@ -15,7 +15,7 @@ buildPythonPackage rec {
 
  checkPhase = ''
  pushd tests
- py.test ./.
+ py.test -Wignore::DeprecationWarning ./.
  popd
  '';