Fix typo, 'which' repeated twice

Author: thomasmulvaney

src/traits/chalk-overview.md

@@ -122,7 +122,7 @@ This rule dictates that `Vec<T>: Clone` is only satisfied if `T: Clone` is also
 satisfied (i.e. "provable").
 
 Similar to [`chalk::program::Program`][chalk-program] which has "rust-like
-things", chalk_ir defines [`ProgramEnvironment`] which which is "pure logic".
+things", chalk_ir defines [`ProgramEnvironment`] which is "pure logic".
 The main field in that struct is `program_clauses`, which contains the
 [`ProgramClause`]s generated by the rules module.