docs: (TanStack#2218)

* Fix typo in Mutations guide

* Update docs/src/pages/guides/mutations.md

Co-authored-by: Dominik Dorfmeister <office@dorfmeister.cc>

* Fix typo in Placeholder Query Data guide

Co-authored-by: Dominik Dorfmeister <office@dorfmeister.cc>

Author: Adebiyi Adedotun

docs/src/pages/guides/placeholder-query-data.md

@@ -12,7 +12,7 @@ Placeholder data allows a query to behave as if it already has data, similar to
 There are a few ways to supply placeholder data for a query to the cache before you need it:
 
 - Declaratively:
-  - Provide `placeholderData` to a query to prepopulate the its cache if empty
+  - Provide `placeholderData` to a query to prepopulate its cache if empty
 - Imperatively:
   - [Prefetch or fetch the data using `queryClient` and the `placeholderData` option](./prefetching)