Remove --keep-file-descriptors flag from systemd docs (#3248)

It has been made the default from bundler-v2.3.0:

rubygems/rubygems#3254
rubygems/rubygems#4812 

[ci skip]

docs/systemd.md

@@ -51,7 +51,7 @@ ExecStart=/<FULLPATH>/bin/puma -C <YOUR_APP_PATH>/puma.rb
 # Variant: Rails start.
 # ExecStart=/<FULLPATH>/bin/puma -C <YOUR_APP_PATH>/config/puma.rb ../config.ru
 
-# Variant: Use `bundle exec --keep-file-descriptors puma` instead of binstub
+# Variant: Use `bundle exec puma` instead of binstub
 # Variant: Specify directives inline.
 # ExecStart=/<FULLPATH>/puma -b tcp://0.0.0.0:9292 -b ssl://0.0.0.0:9293?key=key.pem&cert=cert.pem
 
@@ -76,9 +76,7 @@ compatible with both clustered mode and application preload.
 
 **Note:** Any wrapper scripts which `exec`, or other indirections in `ExecStart`
 may result in activated socket file descriptors being closed before reaching the
-puma master process. For example, if using `bundle exec`, pass the
-`--keep-file-descriptors` flag. `bundle exec` can be avoided by using a `puma`
-executable generated by `bundle binstubs puma`. This is tracked in [#1499].
+puma master process.
 
 **Note:** Socket activation doesn't currently work on JRuby. This is tracked in
 [#1367].