adjust code fences to f90

Throughout the source file (and equally in the mediawiki file),
the label of the code fences was `fortran`.  Because the file is
about Fortran 95, it was updated by

```bash
sed 's/``` fortran/```f90/' f95_features.md > temp.txt
mv temp.text f95_features.md
rm temp.txt
```

Signed-off-by: Norwid Behrnd <[email protected]>

Author: nbehrnd