[lldb] Fix Doxygen warning in SBTrace.h (#170394)
Remove errant `\a` command before `<directory>` in `SaveToDisk`
documentation. The `\a` Doxygen command expects a word argument, but
`<directory>` starts with `<` which Doxygen interprets as HTML. This
fixes:
```
llvm-project/lldb/include/lldb/API/SBTrace.h:60:
Warning 564: Error parsing Doxygen command a: No word followed the command. Command ignored.
```
