fixed typo

see https://github.com/propelorm/Propel2/pull/433#commitcomment-3907618
Merge pull request #442 from staabm/tablediff

fixed typo
[Generator] Decoupled the Model and XML schema representation by introducing a new XmlDumper object.
Merge pull request #441 from hhamon/schema_dumper_feature

[Generator] Decoupled the Model and XML schema representation by introdu...
[Generator] [Builder] fixed syntax errors in builders
Fixed `getPlainSelectedColumns` so it returns no column aliases with aggregate functions.
Merge pull request #444 from marcj/master

Fixed `getPlainSelectedColumns` so it returns no column aliases with agg...
Merge pull request #443 from hhamon/fix_syntax_errors

[Generator] [Builder] fixed syntax errors in builders
[Generator] [Model] added unit tests for the TableDiff class and made some refactoring in the diff API.
[Generator] [Model] fixed phpdoc
Merge pull request #445 from hhamon/diff_testing_cleanup

[Generator] [Model] added unit tests for the TableDiff class and made so...
Made some cleanup (dead code) and small optimizations
[Generator] [Behavior] simplified code
Merge pull request #447 from hhamon/remove_dead_code

Made some cleanup (dead code) and small optimizations
Merge pull request #440 from mpscholten/master

Fixed some phpdoc; gave hasVirtualColumn the expected fluid behaviour; fixed whitespaces before class-keyword
Moved definition of addClassOpen, addClassBody and addClassClose into abstract methods of AbstractOMBuilder