Update README.md

Fixed typo

README.md

 # particless Dynare module
 
 This package, used in [Dynare](http://www.dynare.org), provides
-nonlinear filtering algorithmss. The documentation is available in the
+nonlinear filtering algorithms. The documentation is available in the
 [doc](https://github.com/DynareTeam/particles/tree/master/doc)
-subfolder.
\ No newline at end of file
+subfolder.