Don't perform operation twice

This is why I shouldn't release things at this time of the day.

Signed-off-by: Luís Cobucci <lcobucci@gmail.com>

src/FrozenClock.php

@@ -41,7 +41,7 @@ public function adjustTime(string $modifier): void
             throw new InvalidArgumentException('The given modifier is invalid');
         }
 
-        $this->now = $this->now->modify($modifier);
+        $this->now = $modifiedTime;
     }
 
     public function now(): DateTimeImmutable