minor: fix math expression for L1Ball #287

	name: PyProx

	on: [push, pull_request]

	jobs:
	build:

	runs-on: ubuntu-latest
	strategy:
	matrix:
	python-version: [3.8]

	steps:
	- uses: actions/checkout@v2
	- name: Set up Python ${{ matrix.python-version }}
	uses: actions/setup-python@v2
	with:
	python-version: ${{ matrix.python-version }}
	- name: Install dependencies
	run: \|
	python -m pip install --upgrade pip
	pip install flake8 pytest
	if [ -f requirements.txt ]; then pip install -r requirements-dev.txt; fi
	- name: Install pyproximal
	run: \|
	pip install .
	- name: Test with pytest
	run: \|
	pytest

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