sql_examples.md: fix commands

create table "friends_of_peppa" (
    "id" integer,
    "name" text not null,
    primary key ("id")
) using memtx distributed by ("id")
option (timeout = 3.0)

sbroad: invalid column: Primary key mustn''t contain nullable columns

insert into "friends_of_peppa" ("id", "name") values (1, "Suzy")

sbroad: relational (id 5) not found

Staging: https://docs.binary.picodata.io/picodata/branch-rosik/fix-sql-examples/