Fix markup for backtick.

This markup is the only way to create a code-voice backtick in markdown,
even though it creates a warning due to DocC's meaning of double
backticks to delimit code voice.  See also that known issue:

    swiftlang#71
    swiftlang/swift-markdown#93

Sources/TSPL/TSPL.docc/ReferenceManual/LexicalStructure.md

@@ -430,7 +430,7 @@ The following tokens are reserved as punctuation
 and can't be used as custom operators:
 `(`, `)`, `{`, `}`, `[`, `]`,
 `.`, `,`, `:`, `;`, `=`, `@`, `#`,
-`&` (as a prefix operator), `->`, `\\``,
+`&` (as a prefix operator), `->`, `` ` ``,
 `?`, and `!` (as a postfix operator).
 
 ## Literals