doc: Fix the description of grdcontour -G option (#681)

Co-authored-by: Wei Ji <[email protected]>

Author: seisman

pygmt/base_plotting.py

@@ -295,7 +295,11 @@ def grdcontour(self, grid, **kwargs):
         {J}
         {R}
         {B}
-        {G}
+        label_placement : str
+            ``[d|f|n|l|L|x|X]params``.
+            The required argument controls the placement of labels along the
+            quoted lines. It supports five controlling algorithms. See
+            :gmt-docs:`grdcontour.html#g` for details.
         {U}
         {V}
         {W}