4.1.1 Parsing - adjust heading titles per comment on PR 451

[maryjom]

@daniel-montalvo I still would prefer the headings to read a little differently, such as either of the following:

• 4.1.1 Parsing (Level A in WCAG 2.1) and 4.1.1 Parsing (Obsolete and removed from WCAG 2.2)
• WCAG 2.1: 4.1.1 Parsing (Level A) and WCAG 2.2: 4.1.1 Parsing (Obsolete and removed)

Having "WCAG" between the number and the SC name reads a bit awkwardly.
Originally posted by @maryjom in #451 (comment)

Noting @mitchellevan's additional comment on this:

Once we've settled those two headings, ideally I would also like to edit the two "Applying..." headings to be consistent, but that shouldn't stop this PR from merging.