Add backticks around DocumentChange::npos.

Author: dconeybe

release_build_files/readme.md

@@ -569,7 +569,7 @@ code.
 
 ### 8.1.0
 -   Changes
-    -   Firestore: Fixed a linker error when DocumentChange::npos was used.
+    -   Firestore: Fixed a linker error when `DocumentChange::npos` was used.
         ([#474](https://github.com/firebase/firebase-cpp-sdk/pull/474)).
 
 ### 8.0.0