fix: typo "alumini" (#39502)

fix typo "alumini"

client/src/components/landing/components/Testimonials.js

@@ -14,7 +14,7 @@ const Testimonials = () => {
   return (
     <div className='testimonials'>
       <h1 className='big-heading text-center'>
-        Here is what our alumini say about freeCodeCamp:
+        Here is what our alumni say about freeCodeCamp:
       </h1>
       <div className='testimonials-row' data-test-label='testimonial-cards'>
         {campers.map((camper, i) => {