docs(serialization): remove extra whitespace

Co-authored-by: Luís Fonseca <luis.h.fonseca@tecnico.ulisboa.pt>

core/samples/data/ser/custom/page.md

@@ -8,7 +8,7 @@ the following aliases:
 
 @snippet data/ser/custom/main.cpp Include
 
-We'll define a serializer that will print the data to the standard  output.
+We'll define a serializer that will print the data to the standard output.
 
 @snippet data/ser/custom/main.cpp Your own serializer