Remove stray hard breaks in the formal grammar (#1)

Confirmed that these are the only two places where this happened by
looking at the summary of the grammar and searching for backslashes
in the rendered output.

As discussed here: https://forums.swift.org/t/74659

Author: amartini51

TSPL.docc/ReferenceManual/Declarations.md

@@ -33,7 +33,7 @@ the term *declaration* covers both declarations and definitions.
 > *declaration* → *subscript-declaration* \
 > *declaration* → *macro-declaration* \
 > *declaration* → *operator-declaration* \
-> *declaration* → *precedence-group-declaration* \
+> *declaration* → *precedence-group-declaration*
 
 ## Top-Level Code
 

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:
 >
@@ -748,7 +748,7 @@ make the same change here also.
 > *declaration* → *extension-declaration* \
 > *declaration* → *subscript-declaration* \
 > *declaration* → *operator-declaration* \
-> *declaration* → *precedence-group-declaration* \
+> *declaration* → *precedence-group-declaration*
 
 > Grammar of a top-level declaration:
 >