Fix tiny typo in custom symbols gallery example (#2074)

Author: michaelgrund

examples/gallery/symbols/custom_symbols.py

@@ -15,7 +15,7 @@
 fig = pygmt.Figure()
 fig.basemap(region=[0, 8, 0, 3], projection="X12c/4c", frame=True)
 
-# define pen and fontstlye for annotations
+# define pen and fontstyle for annotations
 pen = "1p,black"
 font = "15p,Helvetica-Bold"