Merge pull request #1644 from tivervac/patch-1

Fixed typo
merged-on-behalf-of: Sebastian Wilzbach <sebi.wilzbach@gmail.com>

Author: dlang-bot

spec/contracts.dd

@@ -238,7 +238,7 @@ assert(&s);               // check that struct S invariant holds
 ------
 
 	$(P Invariants contain assert expressions, and so when they fail,
-	they throw a $(D AssertError)s.
+	they throw $(D AssertError)s.
 	Class invariants are inherited, that is,
 	any class invariant is implicitly anded with the invariants of its base
 	classes.)