[dcl.fct] Fix incorrectly pluralized grammar terms (#3926)

Author: sdkrystian

source/declarations.tex

@@ -3664,9 +3664,9 @@
 
 \pnum
 An abbreviated function template can have a \grammarterm{template-head}.
-The invented \grammarterm{template-parameters} are
+The invented \grammarterm{template-parameter}{s} are
 appended to the \grammarterm{template-parameter-list} after
-the explicitly declared \grammarterm{template-parameters}.
+the explicitly declared \grammarterm{template-parameter}{s}.
 \begin{example}
 \begin{codeblock}
 template<typename> concept C = /* ... */;