Include SAMPLE command and fix typo

Author: ioanatia

x-pack/plugin/esql/src/main/antlr/lexer/Fork.g4

@@ -13,7 +13,7 @@ DEV_FORK :        {this.isDevVersion()}? 'fork'          -> pushMode(FORK_MODE);
 
 mode FORK_MODE;
 FORK_LP : LP -> type(LP), pushMode(DEFAULT_MODE);
-FORK_RO : RP -> type(RP), popMode, popMode;
+FORK_RP : RP -> type(RP), popMode, popMode;
 FORK_PIPE : PIPE -> type(PIPE), popMode;
 
 FORK_WS : WS -> channel(HIDDEN);

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/parser/EsqlBaseLexer.interp

@@ -3317,6 +3317,7 @@ public void testForkAllCommands() {
                ( ENRICH idx2 on f1 with f2 = f3 )
                ( FORK ( WHERE a:"baz" ) ( EVAL x = [ 1, 2, 3 ] ) )
                ( COMPLETION a = b WITH c )
+               ( SAMPLE 0.99 )
             | KEEP a
             """;