Correct the path to the USERS_GUIDE.md

USERS_GUIDE.md is now under ./documentation

README.md

@@ -130,7 +130,7 @@ the possible configure options.
 
 ## Using *ELPA*
 
-Please have a look at the [USERS_GUIDE](USERS_GUIDE.md) file, to get a documentation or at the [online](http://elpa.mpcdf.mpg.de/html/Documentation/ELPA-2021.05.001.rc1/html/index.html) doxygen documentation, where you find the definition of the interfaces. You might want to have a look at the [PERFORMANCE tuning document](./documentation/PERFORMANCE_TUNING.md) to avoid some usual pitfalls.
+Please have a look at the [USERS_GUIDE](./documentation/USERS_GUIDE.md) file, to get a documentation or at the [online](http://elpa.mpcdf.mpg.de/html/Documentation/ELPA-2021.05.001.rc1/html/index.html) doxygen documentation, where you find the definition of the interfaces. You might want to have a look at the [PERFORMANCE tuning document](./documentation/PERFORMANCE_TUNING.md) to avoid some usual pitfalls.
 
 ## Contributing to *ELPA*