Highlight arguments for "style" attribute of "pen" parameter in Gallery example "Line styles" (#2520)

Author: yvonnefroehlich

examples/gallery/lines/linestyles.py

@@ -8,9 +8,9 @@
 
 A *pen* in GMT has three attributes: *width*, *color*, and *style*.
 The *style* attribute controls the appearance of the line.
-Giving "dotted" or "." yields a dotted line, whereas a dashed pen is requested
-with "dashed" or "-". Also combinations of dots and dashes, like ".-" for a
-dot-dashed line, are allowed.
+Giving ``"dotted"`` or ``"."`` yields a dotted line, whereas a dashed pen is
+requested with ``"dashed"`` or ``"-"``. Also combinations of dots and dashes,
+like ``".-"`` for a dot-dashed line, are allowed.
 
 For more advanced *pen* attributes, see the GMT cookbook
 :gmt-docs:`cookbook/features.html#wpen-attrib`.