use backticks for code blocks

Author: ashnazg

proposed/clock.md

@@ -46,7 +46,7 @@ $timestamp = $clock->now()->getTimestamp();
 The clock interface defines the most basic operations to read the current time and date from the clock. 
 It MUST return the time as a `\DateTimeImmutable`.
 
-~~~php
+```php
 <?php
 
 namespace Psr\Clock;
@@ -59,4 +59,4 @@ interface ClockInterface
     public function now(): \DateTimeImmutable;
 
 }
-~~~
+```