doc: Fix some misspellings

doc/developer-notes.md

@@ -654,7 +654,7 @@ Strings and formatting
 
   - Do not use it to convert to `QString`. Use `QString::fromStdString()`.
 
-    - *Rationale*: Qt has build-in functionality for converting their string
+    - *Rationale*: Qt has built-in functionality for converting their string
       type from/to C++. No need to roll your own.
 
   - In cases where do you call `.c_str()`, you might want to additionally check that the string does not contain embedded '\0' characters, because