Apply suggestions from code review

Co-authored-by: Meghan Jones <meghanj@alum.mit.edu>

Author: willschlitzer

pygmt/src/grdgradient.py

@@ -24,7 +24,7 @@
     N="normalize",
     Q="norm_control",
     R="region",
-    S="slopefile",
+    S="slope_file",
     V="verbose",
     n="interpolation",
 )
@@ -123,7 +123,7 @@ def grdgradient(grid, **kwargs):
         grid output is not  needed for this run then do not specify `outgrid`.
         For  subsequent runs,  just use **r** to read these values.  Using
         **R**  will read then delete the statistics file.
-    slopefile : str
+    slope_file : str
         Name of output grid file with scalar magnitudes of gradient vectors.
         Requires `direction` but makes `outgrid` optional.
     {R}