[lex.ccon][except.spec] Remove trailing linebreak from grammar

There are two grammar entries that end with a trailing `\br`
in their specification.  This trailing linebreak does not
seem to be intended, showing up only as a wider spacing
between adjecent grammars --- most notably in Annex A where
otherwise non-adject grammars pick up neighbours.

Author: AlisdairM

source/exceptions.tex

@@ -751,7 +751,7 @@
 \begin{bnf}
 \nontermdef{noexcept-specifier}\br
     \keyword{noexcept} \terminal{(} constant-expression \terminal{)}\br
-    \keyword{noexcept}\br
+    \keyword{noexcept}
 \end{bnf}
 
 \pnum

source/lex.tex

@@ -1426,7 +1426,7 @@
     \terminal{\textbackslash} octal-digit\br
     \terminal{\textbackslash} octal-digit octal-digit\br
     \terminal{\textbackslash} octal-digit octal-digit octal-digit\br
-    \terminal{\textbackslash o\{} simple-octal-digit-sequence \terminal{\}}\br
+    \terminal{\textbackslash o\{} simple-octal-digit-sequence \terminal{\}}
 \end{bnf}
 
 \begin{bnf}