Fix typo in FirebirdDialect - function name is 'sign', not 'sing'

Author: hazzik

src/NHibernate/Dialect/FirebirdDialect.cs

@@ -94,7 +94,7 @@ public FirebirdDialect()
 RegisterFunction("log10", new StandardSQLFunction("log10", NHibernateUtil.Double));
 RegisterFunction("pi", new NoArgSQLFunction("pi", NHibernateUtil.Double));
 RegisterFunction("rand", new NoArgSQLFunction("rand", NHibernateUtil.Double));
-RegisterFunction("sing", new StandardSQLFunction("sing", NHibernateUtil.Double));
+RegisterFunction("sign", new StandardSQLFunction("sign", NHibernateUtil.Int32));
 RegisterFunction("sqtr", new StandardSQLFunction("sqtr", NHibernateUtil.Double));
 RegisterFunction("truncate", new StandardSQLFunction("truncate"));
 RegisterFunction("floor", new StandardSafeSQLFunction("floor", NHibernateUtil.Double, 1));