[lldb][NFCI] Minor cleanup of default OptionValue::GetSubValue implementation

[bulbazord]

Summary

This does 2 things:

• Corrects a minor typo (value subvalue -> valid subvalue)
• Removes an unnecessary instance of str().c_str() (creating a temporary std::string from a StringRef just to get a valid null-terminated string).

lldb/include/lldb/Interpreter/OptionValue.h

public:
 
   virtual void AutoComplete(CommandInterpreter &interpreter,
                             CompletionRequest &request);
 
   // Subclasses can override these functions
   virtual lldb::OptionValueSP GetSubValue(const ExecutionContext *exe_ctx,
                                           llvm::StringRef name,
                                           Status &error) const {
-    error.SetErrorStringWithFormat("'%s' is not a value subvalue",
-                                   name.str().c_str());
+    error.SetErrorStringWithFormatv("'{0}' is not a valid subvalue", name);
     return lldb::OptionValueSP();
   }
 
   virtual Status SetSubValue(const ExecutionContext *exe_ctx,
                              VarSetOperationType op, llvm::StringRef name,
                              llvm::StringRef value);
 
   virtual bool IsAggregateValue() const { return false; }