editorconfig / doc_src: trim trailing whitespace

Commit 0893134543 (Added .editorconfig file (#3332) (#3313),
2016-08-25) trimmed trailing whitespace  for Markdown file (which do
have significant trailing whitespace) but ReStructuredText does not,
and none of our Markdown files cares about this, so let's clean up
whitespace always.

doc_src/cmds/begin.rst

@@ -33,10 +33,10 @@ The following code sets a number of variables inside of a block scope. Since the
 
     begin
         set -l PIRATE Yarrr
-    
+
         ...
     end
-    
+
     echo $PIRATE
     # This will not output anything, since the PIRATE variable
     # went out of scope at the end of the block