Use E, not e, for escaping in example docs

From the Department of Nitpicking, be consistent with other escaping
and use 'E' instead of 'e' to escape the string in the example docs
for GET DISAGNOSTICS stack = PG_CONTEXT.

Noticed by Department Chief Magnus Hagander.

doc/src/sgml/plpgsql.sgml

@@ -2804,7 +2804,7 @@ DECLARE
   stack text;
 BEGIN
   GET DIAGNOSTICS stack = PG_CONTEXT;
-  RAISE NOTICE e'--- Call Stack ---\n%', stack;
+  RAISE NOTICE E'--- Call Stack ---\n%', stack;
   RETURN 1;
 END;
 $$ LANGUAGE plpgsql;