Rename sorting directive

`sort_pipfile` , as requested in https://github.com/pypa/pipenv/pull/5965

pipenv/project.py

@@ -1227,7 +1227,7 @@ class Project:
 
         p[category][normalized_name] = entry
 
-        if self.settings.get("sort_alphabetical"):
+        if self.settings.get("sort_pipfile"):
             p[category] = dict(sorted(p[category].items()))
 
         # Write Pipfile.

tests/integration/test_install_basic.py

@@ -565,7 +565,7 @@ def test_category_sorted_alphabetically_with_directive(pipenv_instance_private_p
         with open(p.pipfile_path, "w") as f:
             contents = """
 [pipenv]
-sort_alphabetical = true
+sort_pipfile = true
 
 [packages]
 atomicwrites = "*"