convert-ly rule for tocItem

[Timothy Lanfear]

Applying convert-ly (2.21.5) to this file produces illegal output with 
the first double quote escaped by a backslash

\version "2.20.0"

\tocItem "Score A"
score { { a'1 } }

Output

\version "2.21.2"

\tocItem \markup \"Score A"
\score { { a'1 } }

--
Timothy Lanfear, Bristol, UK.