Fix the error message when IPython is not available (#701)

When IPython is not available, `fig.show()` reports an error:
```
Cannot find IPython. Make sure you have it installed or use 'external=True' to open in an external viewer.
```

The error message is out-of-date. Instead, it should report:
```
Cannot find IPython. Make sure you have it installed or use 'method="external"' to open in an external viewer.
```

Author: seisman

pygmt/figure.py

@@ -286,7 +286,7 @@ def show(self, dpi=300, width=500, method="static"):
                         [
                             "Cannot find IPython.",
                             "Make sure you have it installed",
-                            "or use 'external=True' to open in an external viewer.",
+                            "or use 'method=\"external\"' to open in an external viewer.",
                         ]
                     )
                 )