fix(dart-plugin): wrong source context config field (#14507)

Fixes the name of the source context field

Author: buenaflor

platform-includes/debug-symbols/dart-plugin/dart.flutter.mdx

@@ -28,10 +28,10 @@ sentry:
   # upload_debug_symbols: false
   # ___PRODUCT_OPTION_START___ source-context
   # Enable source context
-  upload_source_context: true
+  upload_sources: true
   # ___PRODUCT_OPTION_END___ source-context
   # ___PRODUCT_OPTION_START___ source-maps
-  # Enable source maps
+  # Enable source maps (only relevant for Flutter Web)
   upload_source_maps: true
   # ___PRODUCT_OPTION_END___ source-maps
 ```

platform-includes/debug-symbols/dart-plugin/dart.mdx

@@ -28,10 +28,10 @@ sentry:
   # upload_debug_symbols: false
   # ___PRODUCT_OPTION_START___ source-context
   # Enable source context
-  upload_source_context: true
+  upload_sources: true
   # ___PRODUCT_OPTION_END___ source-context
   # ___PRODUCT_OPTION_START___ source-maps
-  # Enable source maps
+  # Enable source maps (only relevant for Flutter Web)
   upload_source_maps: true
   # ___PRODUCT_OPTION_END___ source-maps
 ```