Fix grammar in AddX method responsibilities docs (#11014)

The docs said, "executes" but then said "validate inputs", which didn't read smoothly to me. Saying the method "will" "validate inputs" seems like a better fit.

Author: brettcannon

docs/specs/appmodel.md

@@ -931,7 +931,7 @@ This guide describes each pattern and shows a **verbatim Redis example** at the
 
 ## Adding Resources with `AddX(...)`
 
-An `AddX(...)` method executes:
+An `AddX(...)` method will:
 
 1. **Validate inputs** (`builder`, `name`, required arguments).  
 2. **Instantiate** the data-only resource (`new TResource(...)`).