Update painless-walkthrough.asciidoc (#99263)

Simple typo on line # 119 - "writting" -> "writing"

docs/painless/painless-guide/painless-walkthrough.asciidoc

@@ -116,7 +116,7 @@ GET hockey/_search
 `doc['myfield'].value` throws an exception if
 the field is missing in a document.
 
-For more dynamic index mappings, you may consider writting a catch equation
+For more dynamic index mappings, you may consider writing a catch equation
 
 ```
 if (!doc.containsKey('myfield') || doc['myfield'].empty) { return "unavailable" } else { return doc['myfield'].value }