Fix typo "assemble then" -> "assemble them"

README.md

@@ -158,7 +158,7 @@ software like [lex](https://en.wikipedia.org/wiki/Lex_(software)) and
 [yacc](https://en.wikipedia.org/wiki/Yacc). Instead of writing the grammar
 in a separate file and generating the corresponding code, you use very
 small functions with very specific purpose, like "take 5 bytes", or
-"recognize the word 'HTTP'", and assemble then in meaningful patterns
+"recognize the word 'HTTP'", and assemble them in meaningful patterns
 like "recognize 'HTTP', then a space, then a version".
 The resulting code is small, and looks like the grammar you would have
 written with other parser approaches.