Remove stray \ from grammar of a declaration

As with the previous commit, there's nothing after this line, so a hard
break produces an actual backslash in the rendered output.

When commit 0096c2d (Remove unreachable grammar production, 2023-10-02)
removed the grammar for *declarations*, this hard break got left behind.

Co-authored-by: Kyle Murray <[email protected]>
References: 0096c2d

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/SummaryOfTheGrammar.md

@@ -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:
 >