Correct docstring to conform to convention

Add period to docstring, change from double to float and remove
redundant "the".

Author: rohangirishrao

src/imcflibs/imagej/shading.py

@@ -184,19 +184,19 @@ def process_files(files, outpath, model_file, fmt):
 
 def simple_flatfield_correction(imp, sigma=20.0):
     """
-    Performs a simple flatfield correction to a given ImagePlus stack
+    Performs a simple flatfield correction to a given ImagePlus stack.
 
     The function returns a 32-bit corrected flatfield image.
     Parameters
     ----------
     imp : ij.ImagePlus
         The input stack to be projected.
-    sigma: double, default 20.0
-        The sigma value for the Gaussian blur, default = 20.0
+    sigma: float, optional
+        The sigma value for the Gaussian blur, default=20.0
     Returns
     -------
     ij.ImagePlus
-        The 32-bit image result of the flatfield correction
+        The 32-bit image result of flatfield correction
 
     """
     flatfield = imp.duplicate()