[dcl.spec.auto.general] The placeholder type -> A placeholder type (#4909)

"The placeholder type" gives the wrong impression that we're talking
about the case from the previous paragraph. We're not; this is parallel
to the previous paragraph so should use parallel wording.

Author: zygoloid

source/declarations.tex

@@ -1694,7 +1694,7 @@
 \end{note}
 
 \pnum
-The placeholder type can appear with a function declarator in the
+A placeholder type can appear with a function declarator in the
 \grammarterm{decl-specifier-seq}, \grammarterm{type-specifier-seq},
 \grammarterm{conversion-function-id}, or \grammarterm{trailing-return-type},
 in any context where such a declarator is valid. If the function declarator