Ensure invoke lint task & its docs are in sync

Someday the entire Pelican code base will perhaps have Black and isort
formatting applied to it, but it doesn't make sense for the linter to
check that until it's actually done. This changes the `lint` Invoke task
to run Flake8 on changed lines only, which should bring it into line
with both the current Pre-commit and CI configurations.

Also, the docs erroneously stated that `invoke tests` would also check
for code style compliance, which has herein been fixed.

tasks.py

@@ -70,13 +70,11 @@ def isort(c, check=False, diff=False):
 
 @task
 def flake8(c):
-    c.run(f"{VENV_BIN}/flake8 {PKG_PATH} tasks.py")
+    c.run(f"git diff HEAD | {VENV_BIN}/flake8 --diff --max-line-length=88")
 
 
 @task
 def lint(c):
-    isort(c, check=True)
-    black(c, check=True)
     flake8(c)