[lldb] Fix Doxygen warning in SBTrace.h (llvm#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.
```

Author: zfogg

lldb/include/lldb/API/SBTrace.h

@@ -39,7 +39,7 @@ class LLDB_API SBTrace {
   SBTraceCursor CreateNewCursor(SBError &error, SBThread &thread);
 
   /// Save the trace to the specified directory, which will be created if
-  /// needed. This will also create a file \a <directory>/trace.json with the
+  /// needed. This will also create a file <directory>/trace.json with the
   /// main properties of the trace session, along with others files which
   /// contain the actual trace data. The trace.json file can be used later as
   /// input for the "trace load" command to load the trace in LLDB, or for the