Fix up minor typo

Author: dmaclach

Source/OCMock/OCMConstraint.m

@@ -41,7 +41,7 @@ + (instancetype)constraintWithSelector:(SEL)aSelector onObject:(id)anObject
     NSMethodSignature *signature = [anObject methodSignatureForSelector:aSelector];
     if(signature == nil)
         [NSException raise:NSInvalidArgumentException
-                    format:@"Unkown selector %@ used in constraint.", NSStringFromSelector(aSelector)];
+                    format:@"Unknown selector %@ used in constraint.", NSStringFromSelector(aSelector)];
     NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];
     [invocation setTarget:anObject];
     [invocation setSelector:aSelector];