Remove stray \ from grammar of an expression

Because this is the only line in the grammar box, there's no need to
write a hard break after the production.  Including the backslash (\)
like this produces an actual backslash in the rendered output.

Commit 8e713ce (Switch the grammar to use hard breaks, 2023-07-13)
added hard breaks throughout the grammar, for readability.  At that
time, this line had the definition of *expression-list* after it, so it
needed a hard break.  However, when commit 38eb815 (Remove disused
grammar production, 2024-02-15) later removed *expression-list*, the
hard break got left behind, causing this issue.

Co-authored-by: Kyle Murray <[email protected]>
References: 8e713ce
References: 38eb815

Author: amartini51

TSPL.docc/ReferenceManual/Expressions.md

@@ -20,7 +20,7 @@ in the sections below.
 
 > Grammar of an expression:
 >
-> *expression* → *try-operator*_?_ *await-operator*_?_ *prefix-expression* *infix-expressions*_?_ \
+> *expression* → *try-operator*_?_ *await-operator*_?_ *prefix-expression* *infix-expressions*_?_
 
 ## Prefix Expressions
 

TSPL.docc/ReferenceManual/SummaryOfTheGrammar.md

@@ -315,7 +315,7 @@ make the same change here also.
 
 > Grammar of an expression:
 >
-> *expression* → *try-operator*_?_ *await-operator*_?_ *prefix-expression* *infix-expressions*_?_ \
+> *expression* → *try-operator*_?_ *await-operator*_?_ *prefix-expression* *infix-expressions*_?_
 
 > Grammar of a prefix expression:
 >