diff --git a/nipype/interfaces/fsl/model.py b/nipype/interfaces/fsl/model.py
index 8c269caac..15fb36f6c 100644
--- a/nipype/interfaces/fsl/model.py
+++ b/nipype/interfaces/fsl/model.py
@@ -2245,7 +2245,7 @@ class RandomiseInputSpec(FSLCommandInputSpec):
         desc=("carry out Threshold-Free Cluster Enhancement with 2D " "optimisation"),
         argstr="--T2",
     )
-    f_only = traits.Bool(desc="calculate f-statistics only", argstr="--f_only")
+    f_only = traits.Bool(desc="calculate f-statistics only", argstr="--fonly")
     raw_stats_imgs = traits.Bool(
         desc="output raw ( unpermuted ) statistic images", argstr="-R"
     )

diff --git a/nipype/interfaces/fsl/tests/test_auto_Randomise.py b/nipype/interfaces/fsl/tests/test_auto_Randomise.py
index cf816c56d..9b0b74bf2 100644
--- a/nipype/interfaces/fsl/tests/test_auto_Randomise.py
+++ b/nipype/interfaces/fsl/tests/test_auto_Randomise.py
@@ -37,7 +37,7 @@ def test_Randomise_inputs():
             argstr="-S %.2f",
         ),
         f_only=dict(
-            argstr="--f_only",
+            argstr="--fonly",
         ),
         fcon=dict(
             argstr="-f %s",

fsl randomise f_only flag contains typo, can't work
### Summary
when calling randomise with f_only=True, nipypes interfaces appends --f_only instead of --fonly. This is incorrect and prevents the call from working.

The offending line is [here](https://github.com/nipy/nipype/blob/master/nipype/interfaces/fsl/model.py#L2250), where the last bit should be `argstr="--fonly"`
### Actual behavior
appending --fonly to the randomise call.

### Expected behavior

--f_only is appended, breaking the call.

### Script/Workflow details

```
    fsl_randomise = mem.cache(fsl.Randomise)
    randomise_results = fsl_randomise(
        in_file=`fourdimage.nii`,
        mask=`mask.nii`,
        num_perm=10000,
        f_only=True,
        **kwargs
        )
```

### Platform details:

```
{'commit_hash': 'c5ce0e2c9',
 'commit_source': 'installation',
 'networkx_version': '2.5',
 'nibabel_version': '3.2.1',
 'nipype_version': '1.4.2',
 'numpy_version': '1.20.1',
 'pkg_path': '/usr/local/miniconda/lib/python3.7/site-packages/nipype',
 'scipy_version': '1.6.1',
 'sys_executable': '/usr/local/miniconda/bin/python',
 'sys_platform': 'linux',
 'sys_version': '3.7.4 (default, Aug 13 2019, 20:35:49) \n[GCC 7.3.0]',
 'traits_version': '6.2.0'}
```
nipype/interfaces/fsl/tests/test_auto_Randomise.py::test_Randomise_inputs
nipype/interfaces/fsl/tests/test_auto_Randomise.py::test_Randomise_outputs