Werk #11028: Fixed plugins README

Component	Checks & agents
Title	Fixed plugins README
Date	May 6, 2020
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	1.6.0p12 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME) 1.5.0p25 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

The description of the configuration for asynchronous plugins indicated that the folder should be named after a number of 'minutes'. This is wrong and is now correctly described as 'seconds'.

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