Fix repeated LI supported element (UL)

See WordPress#5539 (comment)

Author: sirreal

src/wp-includes/html-api/class-wp-html-processor.php

@@ -105,7 +105,7 @@
  *  - Formatting elements: B, BIG, CODE, EM, FONT, I, PRE, SMALL, STRIKE, STRONG, TT, U, WBR.
  *  - Heading elements: H1, H2, H3, H4, H5, H6, HGROUP.
  *  - Links: A.
- *  - Lists: DD, DL, DT, LI, OL, LI.
+ *  - Lists: DD, DL, DT, LI, OL, UL.
  *  - Media elements: AUDIO, CANVAS, EMBED, FIGCAPTION, FIGURE, IMG, MAP, PICTURE, SOURCE, TRACK, VIDEO.
  *  - Paragraph: BR, P.
  *  - Phrasing elements: ABBR, AREA, BDI, BDO, CITE, DATA, DEL, DFN, INS, MARK, OUTPUT, Q, SAMP, SUB, SUP, TIME, VAR.