[dcl.type.auto.deduct] Change "occurrences" to "occurrence" (cplusplus#7644)

This appears to be a misapplication of N1984, which contains the wording "Obtain `P` from `T`
by replacing the occurrence of `auto` with a new invented type template parameter `U`."

Author: lprv

source/declarations.tex

@@ -2139,7 +2139,7 @@
 shall precede\iref{basic.lookup.general}
 the \grammarterm{placeholder-type-specifier}.
 Obtain \tcode{P} from
-\tcode{T} by replacing the occurrences of
+\tcode{T} by replacing the occurrence of
 \opt{\grammarterm{type-constraint}} \keyword{auto} either with
 a new invented type template parameter \tcode{U} or,
 if the initialization is copy-list-initialization, with