update nginx subdir config to resolve oc(m|s)-provider

signed-off-by Bernhard Ostertag <[email protected]>

After upgrading to 15.0.5 this is shown in the Admin-panel:
```
Your web server is not properly set up to resolve “/ocm-provider/”. This is most likely related to a web server configuration that was not updated to deliver this folder directly. Please compare your configuration against the shipped rewrite rules in “.htaccess” for Apache or the provided one in the documentation for Nginx at it’s documentation page 1. On Nginx those are typically the lines starting with “location ~” that need an update.
Your web server is not properly set up to resolve “/ocs-provider/”. This is most likely related to a web server configuration that was not updated to deliver this folder directly. Please compare your configuration against the shipped rewrite rules in “.htaccess” for Apache or the provided one in the documentation for Nginx at it’s documentation page 1. On Nginx those are typically the lines starting with “location ~” that need an update.
```
I think for runnning nextcloud in a subdirectory of webroot, this rewrite has to be added to the nignx configuration when running nextcloud in a subdirectory of webroot

admin_manual/installation/nginx.rst

@@ -256,7 +256,8 @@ your nginx installation.
         return 301 $scheme://$host/nextcloud/remote.php/dav;
       }
       location ~/(ocm-provider|ocs-provider)/ {
-        return 301 $scheme://$host/nextcloud/$1/;
+      location ~ ^\/(?:ocm-provider|ocs-provider).* {
+        rewrite ^ /nextcloud$request_uri;
       }
 
       location /.well-known/acme-challenge { }