Merge pull request #130 from jsquyres/pr/trivial-README-update

README.md: trivial typo fix

README.md

@@ -13,7 +13,7 @@ can coexist on the same system.
 Most importantly, Spack is simple. It offers a simple spec syntax so
 that users can specify versions and configuration options
 concisely. Spack is also simple for package authors: package files are
-writtin in pure Python, and specs allow package authors to write a
+written in pure Python, and specs allow package authors to write a
 single build script for many different builds of the same package.
 
 See the