Correct docs after subset argument removal

Author: frankwiles

README.md

@@ -97,7 +97,7 @@ To use Health Check Subsets, Specify a subset name and associate it with the rel
         "SUBSETS": {
             "startup-probe": ["MigrationsHealthCheck", "DatabaseBackend"],
             "liveness-probe": ["DatabaseBackend"],
-            "<SUBSET_NAME>": ["<Health_Check_Service_Name"]
+            "<SUBSET_NAME>": ["<Health_Check_Service_Name>"]
         },
         # .....
     }
@@ -192,7 +192,7 @@ Writing a health check is quick and easy:
         #: even if the check errors.
         critical_service = False
 
-        def check_status(self, subset=None):
+        def check_status(self):
             # The test code goes here.
             # You can use `self.add_error` or
             # raise a `HealthCheckException`,

docs/readme.rst

@@ -181,7 +181,7 @@ Writing a health check is quick and easy:
            #: even if the check errors.
            critical_service = False
 
-           def check_status(self, subset=None):
+           def check_status(self):
                # The test code goes here.
                # You can use `self.add_error` or
                # raise a `HealthCheckException`,

health_check/mixins.py

@@ -46,7 +46,7 @@ def filter_plugins(self, subset=None):
 
         health_check_subsets = HEALTH_CHECK["SUBSETS"]
         if subset not in health_check_subsets or not self.plugins:
-            raise Http404(f"Specify subset: '{subset}' does not exists.")
+            raise Http404(f"Subset: '{subset}' does not exist.")
 
         selected_subset = set(health_check_subsets[subset])
         return {