Add missing code blocks

docs/standard-library/hash-compare-class.md

@@ -12,8 +12,9 @@ The class template describes an object that can be used by any of the hash assoc
 
 ## Syntax
 
+```cpp
 class hash_compare
-   {
+{
    Traits comp;
    public:
    const size_t bucket_size = 4;
@@ -24,7 +25,8 @@ class hash_compare
    bool operator()(
    const Key& key1,
    const Key& key2) const;
-   };
+};
+```
 
 ## Remarks
 

docs/standard-library/input-iterator-tag-struct.md

@@ -12,7 +12,9 @@ A class that provides a return type for `iterator_category` function that repres
 
 ## Syntax
 
+```cpp
 struct input_iterator_tag {};
+```
 
 ## Remarks
 

docs/standard-library/is-placeholder-class.md

@@ -12,9 +12,11 @@ Tests if type is a placeholder.
 
 ## Syntax
 
+```cpp
 struct is_placeholder {
    static const int value;
 };
+```
 
 ## Remarks
 

docs/standard-library/once-flag-structure.md

@@ -11,10 +11,12 @@ Represents a **`struct`** that is used with the template function [call_once](..
 
 ## Syntax
 
+```cpp
 struct once_flag
-   {
+{
    constexpr once_flag() noexcept;
-   };
+};
+```
 
 ## Remarks
 

docs/standard-library/output-iterator-tag-struct.md

@@ -12,7 +12,9 @@ A class that provides a return type for `iterator_category` function that repres
 
 ## Syntax
 
+```cpp
 struct output_iterator_tag {};
+```
 
 ## Remarks