[chore][receiver/nginx] Update README for consistency (open-telemetry#34923)

**Description:** <Describe what has changed.>
<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
This just cleans up the README a bit:
1. Make case of `NGINX` consistent. Happy to change it to `nginx` if
that's preferred.
2. Remove empty `Details` section
3. Cleanup description of the `collection_interval` config option. This
is the same config option many other receivers have, so I don't think
the concept needs explained in-depth here.
4. Remove disclaimer that the component's status is beta and
configuration is subject to change. The README's header states this
component is beta, so I don't think it needs to be repeated.

Author: crobert-1

receiver/nginxreceiver/README.md

@@ -1,4 +1,4 @@
-# Nginx Receiver
+# NGINX Receiver
 
 <!-- status autogenerated section -->
 | Status        |           |
@@ -12,34 +12,27 @@
 [contrib]: https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol-contrib
 <!-- end autogenerated section -->
 
-This receiver can fetch stats from a Nginx instance using the `ngx_http_stub_status_module` module's `status` endpoint.
-
-## Details
+This receiver can fetch stats from a NGINX instance using the `ngx_http_stub_status_module` module's `status` endpoint.
 
 ## Configuration
 
-### Nginx Module
+### NGINX Module
+
 You must configure NGINX to expose status information by editing the NGINX
 configuration.  Please see
 [ngx_http_stub_status_module](http://nginx.org/en/docs/http/ngx_http_stub_status_module.html)
 for a guide to configuring the NGINX stats module `ngx_http_stub_status_module`.
 
 ### Receiver Config
 
-> :information_source: This receiver is in beta and configuration fields are subject to change.
-
 The following settings are required:
 
-- `endpoint` (default: `http://localhost:80/status`): The URL of the nginx status endpoint
+- `endpoint` (default: `http://localhost:80/status`): The URL of the NGINX status endpoint
 
 The following settings are optional:
 
-- `collection_interval` (default = `10s`): This receiver runs on an interval.
-Each time it runs, it queries nginx, creates metrics, and sends them to the
-next consumer. The `collection_interval` configuration option tells this
-receiver the duration between runs. This value must be a string readable by
-Golang's `ParseDuration` function (example: `1h30m`). Valid time units are
-`ns`, `us` (or `µs`), `ms`, `s`, `m`, `h`.
+- `collection_interval` (default = `10s`): This receiver collects metrics on an interval. This value must be a string readable by Golang's [time.ParseDuration](https://pkg.go.dev/time#ParseDuration). Valid time units are `ns`, `us` (or `µs`), `ms`, `s`, `m`, `h`.
+
 - `initial_delay` (default = `1s`): defines how long this receiver waits before starting.
 
 Example: