DOC: Fix typo in note for colorbar fontsize scaling (#4139)

Author: yvonnefroehlich

pygmt/src/colorbar.py

@@ -52,7 +52,7 @@ def colorbar(
     .. note::
        For GMT >=6.5.0, the fontsizes of the colorbar x-label, x-annotations,
        and y-label are scaled based on the width of the colorbar following
-       :math:`\sqrt{{colorbar\_width / 15}}`). To set a desired fontsize via the
+       :math:`\sqrt{{colorbar\_width / 15}}`. To set a desired fontsize via the
        GMT default parameters :gmt-term:`FONT_ANNOT_PRIMARY`,
        :gmt-term:`FONT_ANNOT_SECONDARY`, and :gmt-term:`FONT_LABEL` (or jointly
        :gmt-term:`FONT`) users have to divide the desired fontsize by the value