Fix indentation of bit_color docstring

Author: weiji14

pygmt/base_plotting.py

@@ -414,13 +414,13 @@ def grdimage(self, grid, **kwargs):
             columns) as the input file. Specify *i* to use the PostScript image
             operator to interpolate the image at the device resolution.
         bit_color : str
-           ``color[+b|f]``.
-             This option only applies when a resulting 1-bit image otherwise
-             would consist of only two colors: black (0) and white (255). If
-             so, this option will instead use the image as a transparent mask
-             and paint the mask with the given color. Append **+b** to paint
-             the background pixels (1) or **+f** for the foreground pixels
-             [Default].
+            ``color[+b|f]``.
+            This option only applies when a resulting 1-bit image otherwise
+            would consist of only two colors: black (0) and white (255). If so,
+            this option will instead use the image as a transparent mask and
+            paint the mask with the given color. Append **+b** to paint the
+            background pixels (1) or **+f** for the foreground pixels
+            [Default].
         {J}
         monochrome : bool
             Force conversion to monochrome image using the (television) YIQ