Singular not plural (#20751)

* Singular not plural

Obvious correction. sorry I dont know what I did to display the error.

* Update en-GB.mod_sampledata.ini

administrator/language/en-GB/en-GB.mod_sampledata.ini

@@ -5,6 +5,6 @@
 
 MOD_SAMPLEDATA="Sample Data"
 MOD_SAMPLEDATA_CONFIRM_START="Proceeding will install a sample data set into your Joomla website. This process can't be reverted once done."
-MOD_SAMPLEDATA_INVALID_RESPONSE="There is an error in the sample data plugins. Response is invalid."
+MOD_SAMPLEDATA_INVALID_RESPONSE="There is an error in a sample data plugin. Response is invalid."
 MOD_SAMPLEDATA_ITEM_ALREADY_PROCESSED="This sample data set is already installed."
 MOD_SAMPLEDATA_XML_DESCRIPTION="This Module allows to install sample data."