README.md: trivial typo fix

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

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