removed duplicated sentence fragment

Author: grobmeier

src/site/antora/modules/ROOT/pages/manual/index.adoc

@@ -30,7 +30,6 @@ version, including full-source code, class files and documentation can
 be found at
 https://logging.apache.org/log4j/2.x/index.html[*https://logging.apache.org/log4j/2.x/index.html*].
 
-Inserting log statements into code is a low-tech method for debugging
 Inserting log statements into code is a low-tech method for debugging
 it. It may also be the only way because debuggers are not always
 available or applicable. This is usually the case for multithreaded