Update perspective doc

Author: willschlitzer

pygmt/helpers/decorators.py

@@ -73,7 +73,8 @@
         - 'l' for bilinear
         - 'n' for nearest-neighbor""",
     "p": r"""perspective : list or str
-    ``'[x|y|z]azim[/elev[/zlevel]][+wlon0/lat0[/z0]][+vx0/y0]'``.
+    [**x**\|\ **y**\|\ **z**]\ *azim*\[/\ *elev*\[/\ *zlevel*\]]
+    [**+w**\ *lon0*\/\ *lat0*\[/\ *z0*\]][**+v**\ *x0*/*y0*].
     Select perspective view and set the azimuth and elevation angle of
     the viewpoint. Default is [180, 90]. Full documentation is at
     :gmt-docs:`gmt.html#perspective-full`.""",