Fix invalid mbstring config reference

Closes GH-7637.

[ci skip]

Author: Petar Obradović

php.ini-development

@@ -1675,8 +1675,8 @@ zend.assertions = 1
 
 ; This directive specifies the regex pattern of content types for which mb_output_handler()
 ; is activated.
-; Default: mbstring.http_output_conv_mimetype=^(text/|application/xhtml\+xml)
-;mbstring.http_output_conv_mimetype=
+; Default: mbstring.http_output_conv_mimetypes=^(text/|application/xhtml\+xml)
+;mbstring.http_output_conv_mimetypes=
 
 ; This directive specifies maximum stack depth for mbstring regular expressions. It is similar
 ; to the pcre.recursion_limit for PCRE.

php.ini-production

@@ -1677,8 +1677,8 @@ zend.assertions = -1
 
 ; This directive specifies the regex pattern of content types for which mb_output_handler()
 ; is activated.
-; Default: mbstring.http_output_conv_mimetype=^(text/|application/xhtml\+xml)
-;mbstring.http_output_conv_mimetype=
+; Default: mbstring.http_output_conv_mimetypes=^(text/|application/xhtml\+xml)
+;mbstring.http_output_conv_mimetypes=
 
 ; This directive specifies maximum stack depth for mbstring regular expressions. It is similar
 ; to the pcre.recursion_limit for PCRE.