docs: remove 'alpha' notice from custom health check http headers feature (#623)

Author: Integralist

docs/resources/service_vcl.md

@@ -489,7 +489,7 @@ Optional:
 
 - `check_interval` (Number) How often to run the Healthcheck in milliseconds. Default `5000`
 - `expected_response` (Number) The status code expected from the host. Default `200`
-- `headers` (Set of String) Custom health check HTTP headers (e.g. if your health check requires an API key to be provided). This feature is part of an alpha release, which may be subject to breaking changes and improvements over time
+- `headers` (Set of String) Custom health check HTTP headers (e.g. if your health check requires an API key to be provided).
 - `http_version` (String) Whether to use version 1.0 or 1.1 HTTP. Default `1.1`
 - `initial` (Number) When loading a config, the initial number of probes to be seen as OK. Default `3`
 - `method` (String) Which HTTP method to use. Default `HEAD`

fastly/block_fastly_service_healthcheck.go

@@ -64,7 +64,7 @@ func (h *HealthCheckServiceAttributeHandler) GetSchema() *schema.Schema {
 						Type: schema.TypeString,
 					},
 					Optional:    true,
-					Description: "Custom health check HTTP headers (e.g. if your health check requires an API key to be provided). This feature is part of an alpha release, which may be subject to breaking changes and improvements over time",
+					Description: "Custom health check HTTP headers (e.g. if your health check requires an API key to be provided).",
 				},
 				"host": {
 					Type:        schema.TypeString,