Fixed a typo

I removed the letter "s" from the phrase "uses a custom parsers" so that it now reads "uses a custom parser"

README.rst

@@ -578,7 +578,7 @@ The reasons are:
   process actually modifies the input.
 
 Unable to use the regular parsing techniques, the browser utilizes a custom
-parsers for parsing HTML. The parsing algorithm is described in
+parser for parsing HTML. The parsing algorithm is described in
 detail by the HTML5 specification.
 
 The algorithm consists of two stages: tokenization and tree construction.