Add list of filter names to docstring for recon's FBP and FDK (#1528)

* Add list of filter names to docstring

* Update type of quote used

Signed-off-by: Laura Murgatroyd <60604372+lauramurgatroyd@users.noreply.github.com>

---------

Signed-off-by: Laura Murgatroyd <60604372+lauramurgatroyd@users.noreply.github.com>

Wrappers/Python/cil/recon/FBP.py

@@ -331,7 +331,7 @@ class FDK(GenericFilteredBackProjection):
         A description of the area/volume to reconstruct
     
     filter : string, numpy.ndarray, default='ram-lak'
-        The filter to be applied. Can be a string from: 'ram-lak' or a numpy array.
+        The filter to be applied. Can be a string from: {'`ram-lak`', '`shepp-logan`', '`cosine`', '`hamming`', '`hann`'}, or a numpy array.
 
     Example
     -------
@@ -430,7 +430,7 @@ class FBP(GenericFilteredBackProjection):
         A description of the area/volume to reconstruct
         
     filter : string, numpy.ndarray, default='ram-lak'
-        The filter to be applied. Can be a string from: 'ram-lak' or a numpy array.
+        The filter to be applied. Can be a string from: {'`ram-lak`', '`shepp-logan`', '`cosine`', '`hamming`', '`hann`'}, or a numpy array.
 
     backend : string
         The backend to use, can be 'astra' or 'tigre'. Data must be in the correct order for requested backend.