Update outdated description and summary in gemspec (puma#3328)

puma.gemspec

@@ -4,8 +4,13 @@ Gem::Specification.new do |s|
   s.name = "puma"
   s.version = Puma::Const::PUMA_VERSION
   s.authors = ["Evan Phoenix"]
-  s.description = "Puma is a simple, fast, threaded, and highly parallel HTTP 1.1 server for Ruby/Rack applications. Puma is intended for use in both development and production environments. It's great for highly parallel Ruby implementations such as Rubinius and JRuby as well as as providing process worker support to support CRuby well."
-  s.summary = "Puma is a simple, fast, threaded, and highly parallel HTTP 1.1 server for Ruby/Rack applications"
+  s.description = <<~DESCRIPTION
+    Puma is a simple, fast, multi-threaded, and highly parallel HTTP 1.1 server
+    for Ruby/Rack applications. Puma is intended for use in both development and
+    production environments. It's great for highly parallel Ruby implementations such as
+    JRuby and TruffleRuby as well as as providing process worker support to support CRuby well.
+  DESCRIPTION
+  s.summary = "A Ruby/Rack web server built for parallelism."
   s.email = ["evan@phx.io"]
   s.executables = ["puma", "pumactl"]
   s.extensions = ["ext/puma_http11/extconf.rb"]