Revert wrong fix about --no-merges

There was a fix in 6d9867f
"not merges" -> "not merged" that was wrong. "not merges" was
about flag --no-merges. Then several commits rephrased this
sentence a bit, but original line was right.

I've changed "not merges" -> "not merge commits" so that
this doesn't happen again.

Author: hedrok

book/02-git-basics/sections/viewing-history.asc

@@ -273,7 +273,7 @@ In <<limit_options>> we'll list these and a few other common options for your re
 | `-S`                  | Only show commits adding or removing code matching the string
 |================================
 
-For example, if you want to see which commits modifying test files in the Git source code history are merged and were committed by Junio Hamano in the month of October 2008, you can run something like this:(((log filtering)))
+For example, if you want to see which commits modifying test files in the Git source code history were committed by Junio Hamano in the month of October 2008 and are not merge commits, you can run something like this:(((log filtering)))
 
 [source,console]
 ----