Few documentations fixes. [ci skip] [changelog skip] (#2619)

Sanitizing docs on few places. Ain't much. But, it's honest work :).

docs/deployment.md

@@ -16,7 +16,7 @@ Welcome back!
 
 ## Single vs Cluster mode
 
-Puma was originally conceived as a thread-only webserver, but grew the ability to
+Puma was originally conceived as a thread-only web server, but grew the ability to
 also use processes in version 2.
 
 To run `puma` in single mode (e.g. for a development environment) you will need to

docs/plugins.md

@@ -34,5 +34,5 @@ functionality to augment puma.
 `config` runs when the server is being configured and is passed a `Puma::DSL`
 object that can be used to add additional configuration.
 
-Any public methods in `Puma::Plugin` are the public API that any plugin may
+Any public methods in [`Puma::Plugin`](../lib/puma/plugin.rb) are the public API that any plugin may
 use.

docs/systemd.md

@@ -8,7 +8,7 @@ useful features for running Puma in production.
 ## Service Configuration
 
 Below is a sample puma.service configuration file for systemd, which
-can be copied or symlinked to /etc/systemd/system/puma.service, or if
+can be copied or symlinked to `/etc/systemd/system/puma.service`, or if
 desired, using an application or instance specific name.
 
 Note that this uses the systemd preferred "simple" type where the