milou: fix invalid escape sequence '\$'

[wdconinc]

Briefly, what does this PR introduce?

This PR fixes the following python warning:

$ spack info milou
==> Warning: invalid escape sequence '\$'
==> Warning: invalid escape sequence '\$'
MakefilePackage:   milou

Description:
    MILOU is a Monte Carlo generator for deeply virtual Compton scattering
    (DVCS).

What kind of change does this PR introduce?

• Bug fix (issue #__)
• New feature (issue #__)
• Documentation update
• Other: __

Please check if this PR fulfills the following:

• Tests for the changes have been added
• Documentation has been added / updated
• Changes have been communicated to collaborators

Does this PR introduce breaking changes? What changes might users need to make to their code?

No.

Does this PR change default behavior?

No.