Mention self._vcov in docs (#527)

* feols.vcov() returns covariance matrix

* changed self._vcov to self.vcov in feols.py

* Update quickstart.ipynb with vcov attribute

* Update lpdid.py

rename `self._vcov` to `self.vcov` in the `lpdid` class

* Update quickstart.ipynb

the vcov attribute is always available =)

* Update feols_.py

replacing .vcov with ._vcov

* Update quickstart.ipynb

.vcov to ._vcov

* .vcov to ._vcov

---------

Co-authored-by: Alexander Fischer <alexander-fischer1801@t-online.de>

docs/quickstart.ipynb

@@ -6,7 +6,7 @@
    "source": [
     "# Getting Started with PyFixest\n",
     "\n",
-    "## What is a fix effect model?\n",
+    "## What is a fixed effect model?\n",
     "\n",
     "A fixed effect model is a statistical model that includes fixed effects, which are parameters that are estimated to be constant across different groups. \n",
     "\n",
@@ -719,6 +719,13 @@
     "fit.vcov({\"CRV3\": \"group_id\"}).summary()"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "The estimated covariance matrix is available as an attribute of the `Feols` object called `._vcov`."
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},