Turn 'essid_generators' into a private attribute

The 'essid_generators' attribute is not meant to be used outside the
'WifiESSID' class. As a result, this commit turns it into a private
attribute.

See https://docs.python.org/3/tutorial/classes.html#private-variables.

src/faker_wifi_essid/wifi_essid.py

@@ -43,7 +43,7 @@ def bbox_default_essid(self) -> str:
         return self.hexify("Bbox-^^^^^^", upper=True)
 
     # List of the different ESSID generators.
-    essid_generators: list[Callable] = [
+    _essid_generators: list[Callable] = [
         bbox_default_essid,
         common_essid,
         upc_default_essid,
@@ -54,4 +54,4 @@ def wifi_essid(self) -> str:
         Returns a random fake Wi-Fi essid.
         """
 
-        return self.random_element(self.essid_generators)(self)
+        return self.random_element(self._essid_generators)(self)