Fix their/there grammar nit

src/README.md

@@ -5,7 +5,7 @@ This directory contains the source code of the rust project, including:
 
 For more information on how various parts of the compiler work, see the [rustc guide].
 
-Their is also useful content in the following READMEs, which are gradually being moved over to the guide:
+There is also useful content in the following READMEs, which are gradually being moved over to the guide:
 - https://github.com/rust-lang/rust/tree/master/src/librustc/ty/query
 - https://github.com/rust-lang/rust/tree/master/src/librustc/dep_graph
 - https://github.com/rust-lang/rust/blob/master/src/librustc/infer/region_constraints