Fix missing Concurrency -> Parallelism. (puma#2698)

README.md

@@ -12,7 +12,7 @@
 
 Puma is a **simple, fast, multi-threaded, and highly parallel HTTP 1.1 server for Ruby/Rack applications**.
 
-## Built For Speed & Concurrency
+## Built For Speed & Parallelism
 
 Puma processes requests using a C-optimized Ragel extension (inherited from Mongrel) that provides fast, accurate HTTP 1.1 protocol parsing in a portable way. Puma then serves the request using a thread pool. Each request is served in a separate thread, so truly parallel Ruby implementations (JRuby, Rubinius) will use all available CPU cores.