Update regex.txt

fix typo

Author: oorijsonar

source/reference/operator/query/regex.txt

@@ -23,7 +23,7 @@ Definition
    *strings* in queries. MongoDB uses Perl compatible regular
    expressions (i.e. "PCRE" ) version 8.41 with UTF-8 support.
 
-   To use :query:`$regex`, use one of the following syntax:
+   To use :query:`$regex`, use one of the following syntaxes:
 
    .. code-block:: javascript