Fix typo (#114)

Makefile

@@ -14,7 +14,7 @@
 #
 
 # We only want to format and lint checked in python files
-CHECKED_IN_PYTHING_FILES := $(shell git ls-files | grep '\.py$$')
+CHECKED_IN_PYTHON_FILES := $(shell git ls-files | grep '\.py$$')
 
 FLAKE8_OPTIONS := --max-line-length 120 --extend-ignore E203,W503
 BLACK_OPTIONS := --line-length 120
@@ -27,22 +27,22 @@ all:
 .PHONY: fmt
 fmt:
 	@echo "Running black fmt..."
-	python -m black $(BLACK_OPTIONS) $(CHECKED_IN_PYTHING_FILES)
-	python -m isort $(ISORT_OPTIONS) $(CHECKED_IN_PYTHING_FILES)
+	python -m black $(BLACK_OPTIONS) $(CHECKED_IN_PYTHON_FILES)
+	python -m isort $(ISORT_OPTIONS) $(CHECKED_IN_PYTHON_FILES)
 
 .PHONY: lint
 lint: flake8 fmt-check
 
 .PHONY: fmt-check
 fmt-check:
 	@echo "Running black+isort fmt check..."
-	python -m black $(BLACK_OPTIONS) --check --diff $(CHECKED_IN_PYTHING_FILES)
-	python -m isort --check --diff $(ISORT_OPTIONS) $(CHECKED_IN_PYTHING_FILES)
+	python -m black $(BLACK_OPTIONS) --check --diff $(CHECKED_IN_PYTHON_FILES)
+	python -m isort --check --diff $(ISORT_OPTIONS) $(CHECKED_IN_PYTHON_FILES)
 
 .PHONY: flake8
 flake8:
 	@echo "Running flake8 lint..."
-	python -m flake8 $(FLAKE8_OPTIONS) $(CHECKED_IN_PYTHING_FILES)
+	python -m flake8 $(FLAKE8_OPTIONS) $(CHECKED_IN_PYTHON_FILES)
 
 .PHONY: clean_pyc
 clean_pyc: