AK: Add missing 'const' in Span

Author: https://github.com/mattco98
Commit: https://github.com/SerenityOS/serenity/commit/af9a7b13748
Pull-request: https://github.com/SerenityOS/serenity/pull/6974
Reviewed-by: https://github.com/awesomekling

AK/Span.h

@@ -195,7 +195,7 @@ public:
         return this->m_values[index];
     }
 
-    ALWAYS_INLINE constexpr T& operator[](size_t index) const
+    ALWAYS_INLINE constexpr const T& operator[](size_t index) const
     {
         return at(index);
     }