Update intro-to-reactivity.mdx (#1165)

Author: geophpherie

src/routes/concepts/intro-to-reactivity.mdx

@@ -7,7 +7,7 @@ order: 1
 
 Reactivity powers the interactivity in Solid applications.
 This programming paradigm refers to a system's ability to respond to changes in data or state automatically.
-With Solid, reactivity is the basis of its design, ensuring application's stay up-to-date with its underlying data.
+With Solid, reactivity is the basis of its design, ensuring applications stay up-to-date with their underlying data.
 
 ## Importance of reactivity