Fix formatting issues with __VA_OPT__.

Author: AnthonyCalandra

CPP20.md

@@ -19,7 +19,7 @@ C++20 includes the following new language features:
 - [lambda capture of parameter pack](#lambda-capture-of-parameter-pack)
 - [char8_t](#char8_t)
 - [constinit](#constinit)
-- [__VA_OPT__](#__VA_OPT__)
+- [\_\_VA\_OPT\_\_](#__VA_OPT__)
 
 C++20 includes the following new library features:
 - [concepts library](#concepts-library)
@@ -450,7 +450,7 @@ constinit const char* c = f(true); // OK
 constinit const char* d = g(false); // ERROR: `g` is not constexpr, so `d` cannot be evaluated at compile-time.
 ```
 
-### `__VA_OPT__`
+### \_\_VA\_OPT\_\_
 Helps support variadic macros by evaluating to the given argument if the variadic macro is non-empty.
 ```c++
 #define F(...) f(0 __VA_OPT__(,) __VA_ARGS__)

README.md

@@ -18,7 +18,7 @@ C++20 includes the following new language features:
 - [lambda capture of parameter pack](#lambda-capture-of-parameter-pack)
 - [char8_t](#char8_t)
 - [constinit](#constinit)
-- [__VA_OPT__](#__VA_OPT__)
+- [\_\_VA\_OPT\_\_](#__VA_OPT__)
 
 C++20 includes the following new library features:
 - [concepts library](#concepts-library)
@@ -554,7 +554,7 @@ constinit const char* c = f(true); // OK
 constinit const char* d = g(false); // ERROR: `g` is not constexpr, so `d` cannot be evaluated at compile-time.
 ```
 
-### `__VA_OPT__`
+### \_\_VA\_OPT\_\_
 Helps support variadic macros by evaluating to the given argument if the variadic macro is non-empty.
 ```c++
 #define F(...) f(0 __VA_OPT__(,) __VA_ARGS__)