`show_doc` renders classes with `def` prefix instead of `class`

## `show_doc` renders classes with `def` prefix instead of `class`

### Description

When using `show_doc()` (from nbdev) on a Python class, the generated documentation incorrectly shows the class signature with `def` instead of `class`.

This problem can be seen in the `fastcore` documentation itself:

https://fastcore.fast.ai/meta.html#fixsigmeta

...which incorrectly displays

```python
def FixSigMeta(
args:VAR_POSITIONAL, kwargs:VAR_KEYWORD
):

```

### To Reproduce

```python
from nbdev.showdoc import show_doc

class MyClass:
def __init__(self, param1: str, param2: int = 10):
"""A simple class"""
self.param1 = param1
self.param2 = param2

show_doc(MyClass)
```

### Expected Output

```python
class MyClass(
param1:str, param2:int=10
)
```

### Actual Output

```python
def MyClass(
param1:str, param2:int=10
)
```

### Root Cause

In `fastcore/docments.py` around line 375, the code determines the prefix for the signature:

```python
prefix = 'async def' if inspect.iscoroutinefunction(o) else 'def'
```

This checks for async functions but always defaults to `'def'` for everything else, including classes. It never checks `inspect.isclass(o)` to use `'class'` as the prefix.

### Suggested Fix

```python
if inspect.isclass(o):
prefix = 'class'
elif inspect.iscoroutinefunction(o):
prefix = 'async def'
else:
prefix = 'def'
```

Start in fastcore/docments.py around line 375, where show_doc-related signature formatting chooses the prefix. Reproduce the issue with the provided show_doc(MyClass) example, then verify the generated signature uses class for classes while preserving the existing function behavior.

Written by the indexing model from the issue text.