Fix two typos in x2sys_cross docstrings (#632)
Fix two typos in x2sys_cross docstrings (#632)

Before the fix, building the documentation gives the following warnings:

```
docstring of pygmt.x2sys_cross:32: WARNING: Inline emphasis start-string without end-string.
docstring of pygmt.x2sys_cross:94: WARNING: Inline literal start-string without end-string.
```
GenericMappingTools/gmt-python,GenericMappingTools/gmt-python