chore: fix comment typo (#6573)

correct "a input" to "an input"

dist/vue.js

@@ -83,7 +83,7 @@ function toString (val) {
 }
 
 /**
- * Convert a input value to a number for persistence.
+ * Convert an input value to a number for persistence.
  * If the conversion fails, return original string.
  */
 function toNumber (val) {