The JSFiddle code snippet shown below is referencing the 4th line after the Body tag.
<div class="col-md-4 col-sm-12 shadow-lg p-5 bg-light">
By using p-5, it adds a padding of x5 to the main box. If you remove p-5 completely, there will be no padding at all, causing your heading to stretch to the edges.
If you decide to remove p-5, you may need to add margin or padding to other inner elements such as forms, as removing p-5 could push those elements to the edge of the box as well.
Below is the full BODY section from the JSFiddle with p-5 padding removed and replaced with p-3 padding for the Sign In Title and Form elements.
<body>
<div class="container-fluid vh-100" style="margin-top:300px">
<div class="" style="margin-top:200px">
<div class="rounded d-flex justify-content-center">
<div class="col-md-4 col-sm-12 shadow-lg bg-light">
<div style="background-color:#193D4C;" class="w-100 p-3">
<h3 class="text-light">Sign In</h3>
</div>
<form action="" class="p-3">
<div class="p-4">
<div class="input-group mb-3">
<span class="input-group-text bg-primary"><i
class="bi bi-person-plus-fill text-white"></i></span>
<input type="text" class="form-control" placeholder="Username">
</div>
<div class="input-group mb-3">
<span class="input-group-text bg-primary"><i
class="bi bi-key-fill text-white"></i></span>
<input type="password" class="form-control" placeholder="password">
</div>
<div class="form-check">
<input class="form-check-input" type="checkbox" value="" id="flexCheckDefault">
<label class="form-check-label" for="flexCheckDefault">
Remember Me
</label>
</div>
<button class="btn btn-primary text-center mt-2" type="submit">
Login
</button>
<p class="text-center mt-5">Don't have an account?
<span class="text-primary">Sign Up</span>
</p>
<p class="text-center text-primary">Forgot your password?</p>
</div>
</form>
</div>
</div>
</div>
</div>
</body>