Fix typo in comment (#94)

lexer.go

diff --git a/lexer.go b/lexer.go
index 3235753..eb4d999 100644
--- a/lexer.go
+++ b/lexer.go
@@ -1,6 +1,6 @@
 // TOML lexer.
 //
-// Written using the principles developped by Rob Pike in
+// Written using the principles developed by Rob Pike in
 // http://www.youtube.com/watch?v=HxaD_trXwRE
 
 package toml